Process for producing acetic acid

ABSTRACT

A process for producing acetic acid comprises a process comprising: (1) carbonylating methanol; (2) separating the reaction mixture into a volatile phase and a less-volatile phase; (3) distilling the volatile phase to forma first overhead rich in a lower boiling component, and an acetic acid stream rich in acetic acid; and at least one step group selected from the group consisting of the following sections (4), (9), and (15): (4) a section for separating impurities from the acetic acid stream to give purified acetic acid, (9) a section for separating the first overhead into a stream rich in acetaldehyde and a stream rich in methyl iodide, and (15) a section for absorption-treating an off-gas from the process with an absorption solvent and forming a carbon monoxide-rich stream and an acetic acid-rich stream. In this process, the concentration of oxygen in a gaseous phase of the process is controlled to less than 7% by volume and/or the concentration of oxygen in a liquid phase of the process is controlled to less than 7×10 −5  g/g, and the formation of iodine is reduced. The process effectively reduces or prevents local corrosion of an inner wall of a process unit and/or line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of copending application Ser. No.15/577,524, filed on Nov. 28, 2017, which is a national phase of PCTInternational Application No. PCT/JP2017/041447 filed on Nov. 17, 2017,which claims the benefit under 35 U.S.C. § 119(a) to Patent ApplicationNo. 2017-020775, filed in Japan on Feb. 7, 2017 and Patent ApplicationNo. 2017-105771, filed in Japan on May 29, 2017, all of which are herebyexpressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to methods for preventing or reducing theformation of corrosive components such as iodine and processes forproducing acetic acid by carbonylation of methanol using the abovemethods.

BACKGROUND ART

Acetic acid is produced industrially by carbonylating methanol in thepresence of water, a rhodium catalyst, a metal iodide, and methyliodide. In the methanol-carbonylation process, acetic acid is purifiedand produced by using process units including a reactor forcarbonylating methanol under a carbon monoxide atmosphere, an evaporatorfor separating the reaction mixture fed from the reactor into a volatilephase and a less-volatile phase, a column (or light end column orsplitter column) for distilling and separating the volatile phase intoat least an overhead and an acetic acid stream, and a dehydration columnfor separating water from the acetic acid stream. If necessary, aceticacid is produced by further using a column (or heavy end column) forseparating higher boiling point impurities and/or a production columnfollowing the dehydration column.

Regarding the methanol-carbonylation process, Japanese PatentApplication Laid-Open Publication No. 2007-526310 (JP-2007-526310A,Patent Document 1) discloses an improved method for reducing and/orremoving a permanganate reducing compound (PRC), a C₃₋₈carboxylic acid,and a C₂₋₁₂alkyl iodide compound; the method comprises distilling avolatile phase from a reaction mixture to form a first overhead,distilling the first overhead to form a second overhead containingmethyl iodide, dimethyl ether, and the PRC, subjecting the secondoverhead to water extraction twice, and directly or indirectlyintroducing at least a portion of the resulting second raffinate to areaction medium.

Unfortunately, in such a methanol-carbonylation process, corrosion mayoccur in process units and/or lines. Specifically, an inner wall of aprocess unit and/or line is selectively corroded, which may result inpitting corrosion or spot corrosion that forms pores. Moreover, aproduct acetic acid may be colored, lowering in quality.

In the methanol-carbonylation process, it is known that hydrogen iodidecorrodes an inner wall of a process unit and/or line. Japanese PatentApplication Laid-Open Publication No. 2009-501129 (JP-2009-501129A,Patent Document 2) discloses a process for producing acetic acidcomprising: distilling an acetic acid stream containing acetic acid,hydrogen iodide, a lower boiling point component (or lower boilingcomponent), and a higher boiling point component (or higher boilingcomponent) in a first distillation column to form a first lower boilingpoint stream, a first higher boiling point stream, a first side-cutstream containing acetic acid; and distilling the first side-cut streamin a second distillation column to form a second lower boiling pointstream, a second higher boiling point stream, and a second side-cutstream containing acetic acid; wherein water or water and at least onecomponent (A) selected from the group consisting of methanol and methylacetate is fed to the first distillation column to convert hydrogeniodide into a lower boiling component, such as methyl iodide, forremoving hydrogen iodide.

Unfortunately, even after the separation of hydrogen iodide by such aprocess, pitting corrosion or spot corrosion may still occur in theprocess unit or line.

CITATION LIST Patent Literature

Patent Document 1: JP-2007-526310A (Claims)

Patent Document 2: JP-2009-501129A (Claims)

SUMMARY OF INVENTION Technical Problem

It is therefore an object of the present invention to provide a methodfor effectively preventing local corrosion of an inner wall of a processunit and/or line and a process for producing acetic acid.

Another object of the present invention is to provide a method foreffectively reducing coloring of product acetic acid and a process forproducing acetic acid.

It is still another object of the present invention to provide a methodfor preventing coloring of product acetic acid and preventing corrosionby hydrogen iodide and a process for producing acetic acid.

It is a further object of the present invention to provide a method foreffectively preventing corrosion of a process unit and/or line made of alow-grade metal material and a process for producing acetic acid.

Solution to Problem

The inventors of the present invention made intensive studies to achievethe above objects and finally found, in a process for producing aceticacid by carbonylation of methanol, that oxygen enters a process streamby various factors including components introduced in the process fromthe outside, the oxygen oxidizes hydrogen iodide or methyl iodide toform by-product iodine which corrodes a process unit and/or line; andthat control of the concentration of oxygen in the process stream to apredetermined concentration or less effectively reduces the formation ofby-product iodine to prevent the local corrosion of the process unitand/or line. The present invention was accomplished based on the abovefindings.

That is, an aspect of the present invention provides a process forproducing acetic acid, comprising: (1) allowing methanol tocarbonylation react with carbon monoxide (or carbonylating methanol withcarbon monoxide) in the presence of a catalyst system, acetic acid,methyl acetate, and water, wherein the catalyst system contains a metalcatalyst, an ionic metal iodide, and methyl iodide; (2) separating thereaction mixture into a volatile phase and a less-volatile phase; (3)distilling the volatile phase to form a first overhead and an aceticacid stream, wherein the first overhead is rich in at least one lowerboiling component selected from the group consisting of methyl iodideand acetaldehyde, the acetic acid stream is rich in acetic acid; and atleast one section selected from the group consisting of the followingsections (4), (9), and (15):

(4) a purification section for obtaining purified acetic acid from theacetic acid stream;

(9) a separation section for separating at least acetaldehyde from thefirst overhead; and

(15) an off-gas treatment section for absorption-treating an off-gasfrom the process with an absorption solvent and forming or obtaining acarbon monoxide-rich stream and an acetic acid-rich stream.

The purification section (4) may comprise at least (5) a dehydrationstep (preferably at least (5) a dehydration step and (6) a higherboiling component removing step) among the following steps (5) to (8):

(5) dehydrating the acetic acid stream (or removing water from theacetic acid stream),

(6) removing a higher boiling component (or fraction) from the aceticacid stream,

(7) further purification-distilling an acetic acid stream from the step(6), and

(8) ion-exchange separating an iodine compound from an acetic acidstream from the step (7).

The separation section (9) may comprise at least steps (10) to (13)among the following steps (10) to (14):

(10) condensing the first overhead to form two liquid phases with anupper phase and a lower phase (or liquid-liquid separating the firstoverhead by condensation to form an upper phase and a lower phase),

(11) forming a fifth overhead from the upper phase and/or the lowerphase, wherein the fifth overhead is rich in acetaldehyde and methyliodide,

(12) extracting acetaldehyde from the fifth overhead to form an extractand a raffinate, wherein the extract is rich in acetaldehyde and theraffinate is rich in methyl iodide,

(13) separating an aldehyde from the extract and/or the raffinate, and

(14) separating an alkane from the upper phase and/or the lower phase.

The off-gas treatment section (15) may comprise at least one absorptionstep selected from the group consisting of steps (16) and (17) among thefollowing steps (16) to (18):

(16) absorbing the off-gas to an absorption solvent at a high pressure,

(17) absorbing the off-gas to an absorption solvent at a low pressure,and

(18) diffusing a gas (or a gaseous component) absorbed in the absorptionsteps (16) and (17).

In the process for producing acetic acid, the concentration of oxygen iscontrolled (or regulated) in at least one selected from the groupconsisting of the following (a) and (b):

(a) the concentration of oxygen in a gaseous phase (or a gas phase) ofthe process is controlled (or regulated) to less than 7% by volume,

(b) the concentration of oxygen in a liquid phase of the process iscontrolled (or regulated) to less than 7×10⁻⁵ g/g.

The gaseous phase of the process may contain at least one memberselected from the group consisting of methyl iodide and hydrogen iodide.The gaseous phase of the process may further contain at least one memberselected from the group consisting of acetic acid, methyl acetate,methanol, water, acetaldehyde, a by-product derived from acetaldehyde,and a dialkyl ether. The by-product may contain at least one memberselected from the group consisting of an alkyl iodide with 2 or morecarbon atoms, an alkanal with 4 or more carbon atoms, analkanecarboxylic acid with 3 or more carbon atoms, an alkane, and aketone. The dialkyl ether may contain at least dimethyl ether.

In accordance with an aspect of the present invention, in the productionprocess (at least one process stream selected from the group consistingof a stream of a process unit and a stream of a process line), theconcentration of oxygen may be controlled or regulated in at least oneselected from the group consisting of the following (a-1) and (b-1):

(a-1) the concentration of oxygen in the gaseous phase may be controlledto, for example, 5% by volume or less,

(b-1) the concentration of oxygen in the liquid phase may be controlledto, for example, 2×10⁻⁵ g/g or less.

The concentration of oxygen higher than these values may result in theformation of iodine in the process and the corrosion of a process unitor line.

In at least one process stream selected from the group consisting of astream of a process unit and a stream of a process line, the ratio ofoxygen relative to carbon monoxide in each of the gaseous phase and theliquid phase may be 2% by volume or less (for example, 1% by volume orless).

In order to regulate the concentration of oxygen in the gaseous phaseand/or the concentration of oxygen in the liquid phase, at least onecomponent (oxygen source) selected from the group consisting of anoxygen-containing gas, an oxygen-containing compound, and an oxygengenerator may be introduced in the process. In at least one processstream selected from the group consisting of a stream of a process unitand a stream of a process line, the introduction of such a component(oxygen source) may control (or regulate) the concentration of oxygen inthe gaseous phase and/or the concentration of oxygen in the liquid phaseto the following concentration(s):

the concentration of oxygen in the gaseous phase may be controlled (orregulated) to 1 ppt by volume or more (for example, 100 ppt by volume ormore),

the concentration of oxygen in the liquid phase may be controlled (orregulated) to 0.1×10⁻⁹ g/g or more (for example, 0.1×10⁻⁸ g/g or more).

In order to reduce (or prevent) the formation (or production) of iodine,the concentration of oxygen in at least one process stream selected fromthe group consisting of the gaseous phase and the liquid phase may becontrolled (or regulated) to 0.25 mol or less relative to 1 mol of thetotal amount of hydrogen iodide and methyl iodide.

Another aspect of the present invention provides a method for preventingor reducing formation (or production) of iodine in a process. The methodcomprises (1) allowing methanol to carbonylation react with carbonmonoxide (or carbonylating methanol with carbon monoxide) in thepresence of a catalyst system, acetic acid, methyl acetate, and water,wherein the catalyst system contains a metal catalyst, an ionic metaliodide, and methyl iodide; (2) separating the reaction mixture into avolatile phase and a less-volatile phase; (3) distilling the volatilephase to form a first overhead and an acetic acid stream, wherein thefirst overhead is rich in at least one lower boiling component selectedfrom the group consisting of methyl iodide and acetaldehyde, and theacetic acid stream is rich in acetic acid; and at least one sectionselected from the group consisting of the following sections (4), (9),and (15):

(4) a purification section for obtaining purified acetic acid from theacetic acid stream;

(9) a separation section for separating at least acetaldehyde from thefirst overhead; and

(15) an off-gas treatment section for absorption-treating an off-gasfrom the process with an absorption solvent and forming or obtaining acarbon monoxide-rich stream and an acetic acid-rich stream.

In such a method, the concentration of oxygen is controlled (orregulated) in at least one selected from the group consisting of thefollowing (a) and (b), and the formation of iodine is reduced:

(a) the concentration of oxygen in a gaseous phase portion of theprocess is controlled (or regulated) to less than 7% by volume,

(b) the concentration of oxygen in a liquid stream of the process iscontrolled (or regulated) to less than 7×10⁻⁵ g/g.

The gaseous phase (or gaseous phase portion) of the process with thepredetermined oxygen concentration means all gaseous phases of theprocess. The gas forming the gaseous phase (or gaseous phase portion)means an off-gas (all off-gases) from the process, and the gas may be an“off-gas” to be subjected to the off-gas treatment section or may be an“off-gas” from a process unit and/or line (“off-gases” from all processunits and/or lines). The “off-gas” does not necessarily mean a gasdischarged from the process to the outside of the system, but also meansa gas in the process (for example, a gas in a process unit and line).

As used in this description and claims, the term “process unit” means anapparatus or a unit for a unit operation of a process, such as areaction, an evaporation, a distillation, a cooling and/or condensation,a liquid-liquid separation, a holding (storage), or an absorption. Asused in this description and claims, acetaldehyde andacetaldehyde-derived by-products that shorten a permanganate time in apermanganate reducing compound test may simply be referred to as PRC's.Examples of such PRC's may include an aldehyde compound, and an alkyliodide with two or more carbon atoms. Unless otherwise specificallynoted, an acetaldehyde-containing aqueous phase obtainable by aliquid-liquid (or biphasic) separation is synonymous with a light phaseor an upper phase, and a methyl iodide-containing organic phaseobtainable by a liquid-liquid (or biphasic) separation is synonymouswith a heavy phase, a methyl iodide phase, or a lower phase. An aqueousphase obtainable by extraction is synonymous with an extract, and anorganic phase obtainable by extraction is synonymous with a raffinate.

As used in this description and claims, a gaseous phase portion and agaseous stream may correctively be referred to as “gaseous phase”, and aliquid phase portion and a liquid stream may correctively be referred toas “liquid phase”. As used in this description and claims, the totalamount of the mixture forming each of the gaseous phase and the liquidphase, including impurities, is 100%. If the mixture forming the gaseousphase (gaseous mixture) contains a condensable component, thecomposition of the gaseous-phase mixture cannot be measured accuratelyaccording to circumstances. The reason why is as follows: even if themixture is gaseous under a process condition (temperature and pressure),the condensable component in the mixture having a reduced temperature bysampling may be liquefied under a room temperature and an atmosphericpressure (25° C., ≅1 atom 0.1 MPa). Thus, the composition of the mixtureforming the gaseous phase (gaseous mixture) is expressed based on thevolume (% by volume) or weight (% by weight) of the gaseous-phasemixture at a temperature of 25° C. Moreover, the composition of themixture forming the liquid phase (liquid mixture) is expressed based onthe weight (e.g., % by weight).

Advantageous Effects of Invention

According to the present invention, since the concentration of oxygen inthe process stream is controlled (or regulated) to a predeterminedconcentration or less, the formation of by-product iodine is inhibitedand suppressed effectively to reduce or prevent the local corrosion ofthe inner wall of the process unit and/or line. The process also reducesthe total iodine concentration in a product acetic acid to effectivelysuppress or prevent the coloring of the product acetic acid. Further,the process prevents the coloring of the product acetic acid, as well asreduces the formation of hydrogen iodide from iodine and thus preventsthe corrosion by hydrogen iodide. Accordingly, the corrosion of aprocess unit and/or line made of a low-grade metal material is alsoeffectively preventable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram for explaining an example of a process from areaction step through a liquid-liquid separation step and a seconddistillation step in a production process (or production apparatus) ofacetic acid in accordance with an embodiment of the present invention.

FIG. 2 is a flow diagram for explaining an acetic acid purification stepincluding a second distillation step.

FIG. 3 is a flow diagram for explaining a separation step for separatingat least acetaldehyde from a liquid-liquid separation step.

FIG. 4 is a flow diagram for explaining an off-gas treatment step fortreating an off-gas from a process.

DESCRIPTION OF EMBODIMENTS

In a process for producing acetic acid by carbonylation of methanol,oxygen enters a process stream due to various factors, for example,components introduced in the process from the outside. For example,carbon monoxide or methanol is obtainable by partially oxidizing acarbon source (a carbon compound or a hydrocarbon compound), such asfossil fuel (such as coal or petroleum) or natural gas, with oxygen orair to give a syngas (CO, H₂, CO₂, a trace of O₂), and purifying thesyngas; where the method of partially oxidizing the carbon source mayinclude, for example, a steam methane reforming (SMR), an autothermalreforming (ATR), and a partial oxidation (PDX). Not only in the partialoxidation with oxygen but also in the SMR, the carbon source or steamcontains oxygen. Accordingly, a trace of oxygen enters the process byintroduction (or feeding) of carbon monoxide and methanol as rawmaterials to a reactor, or by supply or addition of methanol to aprocess unit (such as a dehydration column or a treatment tank) toconvert hydrogen iodide into methyl iodide for removing hydrogen iodide.

Moreover, to adjust a water content in the process, water is supplied tothe process or water is used in the process. For example, water is fedto a reaction step; or a first overhead from a light end column (asplitter column) for removing a lower boiling component is distilled inan aldehyde-removing column to form a second overhead, and the secondoverhead is subjected to extraction with water (e.g., in a waterextractor or a water extractive distillation column). Further, anaqueous solution of an alkali metal hydroxide may be used to removehydrogen iodide in a process unit such as a dehydration column or atreatment tank. Such water contains a trace of oxygen dissolved therein,and oxygen enters the process stream by the use of such water.

Furthermore, the acetic acid production process by carbonylationincludes, between a reactor and a production column, apparatuses such asvarious tanks or hold tanks, pumps, and instruments and gauges (e.g., alevel gauge and a pressure gauge). In order to prevent the processstream (e.g., an acetic acid stream) from flowing back to theinstruments and gauges and liquefying, or in order to prevent carbonmonoxide from leaking from a stirring shaft of the reactor, nitrogen gaspurge may be performed to a high-pressure seal portion or otherportions. The nitrogen gas purge to the instruments and gauges resultsin introduction of nitrogen gas into the process. In pressure sealing toa seal portion of the stirring shaft, a portion of nitrogen gas may leakin the reactor through the seal portion. Such nitrogen gas also containsa trace of oxygen.

The oxygen introduced into the process due to various factors asdescribed above reacts with hydrogen iodide or methyl iodide present inthe process to generate free iodine I₂ by the oxidative reaction (suchas 2HI+½O₂→I₂+H₂O; 2CH₃I+½O₂→CH₃OCH₃+I₂). In a case where the generatediodine I₂ is attached or adheres to an inner wall of a process unitand/or line, it has been found that the attached portion is selectivelyor locally corroded, which results in pitting corrosion or spotcorrosion that forms pores.

Moreover, in an atmosphere having a water concentration of 5% by weightor less, hydrogen iodide HI usually behaves in the same manner as waterand is condensed in top(s) of a light end column for removing a lowerboiling component, a dehydration column, a heavy end column for removinga higher boiling component, and/or a production column. Whereas, iodineI₂, which has a higher boiling point than hydrogen iodide HI, is carriedtogether with a higher boiling point fraction of the process unit (forexample, a side-cut stream from a light end column for removing a lowerboiling component, a bottom stream from a dehydration column, and aside-cut stream from a production column) to finally enter the productacetic acid. Thus, it has been also found that the product acetic acidmay have an increased iodine concentration or may be colored dark toreddish brown peculiar to iodine I₂. The iodine-contaminated productacetic acid inhibits a catalytic activity in producing an acetic acidderivative such as vinyl acetate. Accordingly, it is usually necessaryto suppress the concentration of iodine in the product acetic acid to asvery low as 10 ppb by weight or less.

Further, as described above, methanol or an alkali metal hydroxide (suchas potassium hydroxide) may be added to a process unit such as adehydration column to convert a trace of hydrogen iodide HI into methyliodide MeI or an alkali iodide (such as KI) which is then removed. Evenin such a method, it is impossible to remove iodine I₂ produced fromhydrogen iodide HI and/or methyl iodide MeI. In a process downstreamside of the process unit such as a dehydration column, the concentrationof hydrogen iodide HI is reducible, while exposure of a process streamcontaining iodine I₂ to a reduction atmosphere generates hydrogen iodideHI by the inverse reaction. Thus, an inner wall of a process unit and/orline made of a low-grade metal material (for example, a low-gradematerial stainless steel (SUS), a HASTELLOY C material) may be corrodednot locally by iodine I₂ but uniformly by hydrogen iodide HI.

According to an embodiment of the present invention, the concentrationof oxygen in the process stream is controlled to solve such problems.Incidentally, the methanol carbonylation process (the reaction system)is usually a pressurized system, and thus the concentration of oxygen inthe process stream is regulatable (or controllable) by controlling theoxygen concentrations in raw materials and respective feed lines. Forexample, the concentration of oxygen in carbon monoxide is controllableby appropriately operating a carbon monoxide production process. Forexample, the concentration of oxygen in carbon monoxide may becontrolled by: regulating (or adjusting) the feed amount of oxygenand/or the feed amount of a steam (water vapor) relative to a carbonmonoxide raw material (such as coal or natural gas, heavy oil, orasphalt) to partially oxidize the raw material with oxygen completely;measuring the concentration of oxygen in a purified carbon monoxide todetermine the advisability of use based on the measured value;feedback-controlling the carbon monoxide production process based on themeasured value to control the concentration of oxygen in carbonmonoxide; or introducing an inactive or inert gas based on the measuredvalue to control the concentration of oxygen in carbon monoxide.

Further, with respect to methanol, the concentration of oxygen dissolvedin methanol may be measured to determine the advisability of use basedon the measured value; or the concentration of dissolved oxygen may becontrolled by heating or other means based on the measured value.Moreover, for water and/or an aqueous solution [an alkaline aqueoussolution (an aqueous solution of an alkali metal hydroxide) or anaqueous solution of sodium hypophosphite] to be fed to the process (suchas the reaction system), the concentration of dissolved oxygen may bemeasured to determine the advisability of use based on the measuredvalue; or there may be used water or an aqueous solution having aconcentration of dissolved oxygen controlled by heating or other meansbased on the measured value (for example, water or an aqueous solutionhaving a reduced oxygen concentration by boiling or other means).

Further, for a gas or liquid to be fed in the process, the concentrationof oxygen may also be measured in the same manner as described above,and the oxygen concentration in the process stream can be adjusted orcontrolled based on the measured value.

Furthermore, the oxygen concentration in the process stream may becontrolled by using a method for minimizing the amount of nitrogen purgegas in the process stream, a method for changing the purge gas to carbonmonoxide purge gas or another inactive purge gas, or other methods.

As an oxygen analyzer determining an oxygen concentration in gas orgaseous phase, there may be used various oxygen analyzers, for example,an explosion-proof magnetic pressure type oxygen analyzer (MPA-51d/pmanufactured by HORIBA, Ltd.), a separate type zirconia oxygen analyzer(ZR402G, ZR22G, manufactured by Yokogawa Electric Corporation), and agas analyzer using tunable diode laser absorption spectroscopy [e.g.,(all-in-one) SITRANS SL manufactured by NOHKEN Inc.; a gas analyzermanufactured by METTLR; and a gas analyzer (O₂ meter) manufactured byIijima Electronics Corporation].

Examples of an oxygen analyzer (a dissolved oxygen sensor) for liquid orliquid phase may include “DO”, “OC”, “ODM” and “OBM” types manufacturedby DKK-TOA Corporation, “DO meter” manufactured by Iijima ElectronicsCorporation, and in addition, an oxygen analyzer (manufactured byMETTLR) that can measure the concentration of oxygen dissolved in waterand a solvent (methanol), and an “OX type” (manufactured by YokogawaElectric Corporation) for measuring the concentration of oxygen in gas.

According to an “0064 type” 7561L model or other models manufactured byDKK-TOA Corporation, the detection limit of oxygen in a liquid is 0.1μg/L. For example, the minimum limit of determination of the oxygenconcentration in a liquid having a specific gravity of 1 is(0.1/1000000) g/1000 g=0.1 ppb, and the detection limit of the oxygenconcentration in a liquid having a specific gravity of 2 is 0.05 ppb.For an “OX400” manufactured by Yokogawa Electric Corporation, theminimum limit of determination of the oxygen concentration in a gas is0.01 vol ppm (10 ppb). For a sample (a gaseous phase or a liquid phase)having an oxygen concentration of less than the minimum limit ofdetermination, the concentration of oxygen in the sample may bedetermined by condensing the gaseous phase or the liquid phase using acommon method (for example, selective adsorption of oxygen to anadsorbent, selective diffusion of oxygen through a permselectivemembrane such as an oxygen-enriched membrane, distillation for forming alight fraction and a heavy fraction, and extraction) to give an oxygenconcentrate, measuring the concentration of oxygen in the concentrate,and converting the measured value into the concentration of oxygen inthe sample.

The concentrations of oxygen in a gas (or a gaseous phase) and a liquid(or a liquid phase) may continuously be observed by monitoring a valuedetected or measured with an oxygen analyzer (an oxygen sensor) disposedto (or installed in) a process unit or a process line or is observableby regularly taking and analyzing a sample from a process unit or aprocess line. Moreover, the oxygen concentration may be controlled bycomparing the detected or measured value with the oxygen analyzer (theoxygen sensor) with an upper reference value (a threshold value), and,in a case where the detected or measured value reaches the thresholdvalue, automatically introducing a fluid (gas or liquid) having a lowoxygen concentration to a process stream or switching the introductionstream to a fluid having a low oxygen concentration. Further, when theoxygen concentration is excessively low (when the concentration ofoxygen reaches a threshold value as a lower reference value), an oxygensource may be introduced to the process stream.

In a process under a reduced pressure system, the oxygen concentrationin the process stream under the reduced pressure system may becontrolled by: controlling the pressure to a predetermined pressure withintroduction of an inactive gas while maintaining an airtight conditionfor holding the operating pressure, then starting the operation, andmeasuring the oxygen concentration in a waste gas from a vacuum pump.

Such a control of the oxygen concentration allows the reduced formationof by-product iodine and thus provides a useful process condition thatsolves problems including local corrosion by iodine, increasedconcentration of total iodine in a product acetic acid, and coloring ofthe product acetic acid. Moreover, the present invention is highlyuseful for controlling the iodine concentration in the product aceticacid to a low concentration as extremely low as 10 ppb by weight orless. Furthermore, it is known that high-grade corrosion-resisted metalssuch as zirconium show a perfect corrosion resistance under awide-ranging condition including a reducing condition and an oxidizingcondition. However, such a high-grade corrosion-resisted metal may becorroded under a strongly oxidizing condition. Thus, depending on theselection of a material of a process unit and/or line, although theprocess unit and/or line may show a corrosion resistance in an extent ofa high oxygen concentration, the process unit and/or line may becorroded according to the concentration of oxygen. The present inventioncan also reduce such corrosion.

As apparent from these matters, the present invention can also beapplied to any process unit (step) and line in a process for producingacetic acid by methanol carbonylation.

The present invention can be applied to a process stream (for example, agaseous phase of a process) containing at least one member selected fromthe group consisting of methyl iodide and hydrogen iodide for reducingthe formation of by-product iodine. Further, as described later, theprocess stream (for example, a gaseous phase of a process) may containat least one member selected from the group consisting of acetic acid,methyl acetate, methanol, water, acetaldehyde, by-products derived fromacetaldehyde, and dialkyl ethers, depending on the process unit and/orthe process line. The by-products may contain at least one memberselected from the group consisting of an alkyl iodide with 2 or morecarbon atoms, an alkanal with 4 or more carbon atoms, analkanecarboxylic acid with 3 or more carbon atoms, an alkane, and aketone. The dialkyl ethers may contain at least dimethyl ether.

According to an embodiment of the present invention, at least one oxygenconcentration selected from the group consisting (a) a concentration ofoxygen in a gaseous phase of a process and (b) a concentration of oxygenin a liquid phase of a process as described later is controlled in aproduction process of acetic acid (at least one process stream selectedfrom the group consisting of a stream of a process unit and a stream ofa process line).

(a) The concentration of oxygen in the gaseous phase of the process iscontrolled to less than 7% by volume, and may be 6.5% by volume or less(for example, 6% by volume or less), preferably 5.5% by volume or less,or may be controlled to usually 5% by volume or less (for example, 3% byvolume or less), preferably 1% by volume or less (for example, 0.5% byvolume or less), more preferably 0.1% by volume or less (for example,0.01% by volume or less), and particularly 0.001% by volume (10 ppm byvolume) or less (for example, 0.0001% by volume (1 ppm by volume) orless).

The concentration of oxygen in the gaseous phase may have any lowerlimit, and may be, for example, 1 ppt by volume or more (for example,100 ppt by volume or more), preferably 1 ppb by volume or more (forexample, 100 ppb by volume or more) or may be zero (0) or a minimumlimit of determination (or measurement) or less.

(b) The concentration of oxygen in the liquid phase and may becontrolled to 2×10⁻⁵ g/g or less (for example, 1×10⁻⁵ g/g or less),preferably 0.5×10⁻⁵ g/g or less (for example, 0.1×10⁵ g/g or less), morepreferably 0.05×10⁻⁵ g/g or less (for example, 0.01×10⁻⁵ g/g or less),and particularly 0.001×10⁻⁵ g/g or less (for example, 0.0001×10⁻⁵ g/g orless).

Moreover, the concentration of oxygen in the liquid phase may have anylower limit, and may be, for example, 0.1×10⁻⁹ g/g or more or may bezero (0) or a minimum limit of determination (or measurement) or less.In a liquid phase such as a process liquid under pressure or ahigh-temperature process liquid, in some cases, the concentration ofoxygen (or the concentration of dissolved oxygen) cannot be measuredaccurately due to difficulty of sampling, vaporization of oxygen, orother factors. In such a case, the concentration of oxygen in theprocess liquid may be measured, as an estimated value (an experimentalestimated value), by determining a concentration of oxygen in theprocess liquid under a plurality of conditions with varying temperaturesand/or pressures to estimate an oxygen concentration at an actualprocess temperature and pressure, or may be calculated using Aspen Plus(manufactured by Aspen Technology, Inc.).

As the concentration of oxygen in the process stream (the gaseous phaseand the liquid phase) is increased, iodine is easily formed in theprocess stream.

Though a lower oxygen concentration is preferred, in a case where theconcentration of oxygen is too low, the corrosion speed of the processunit and/or line may be increased due to an excessively strong reducingatmosphere. Thus, to control the oxygen concentration in the processstream (the gaseous phase and the liquid phase), at least one oxygensource selected from the group consisting of an oxygen-containing gas,an oxygen-containing compound, and an oxygen generator may be introducedto the process to control the oxygen concentration in the gaseous phaseand/or liquid phase in the process stream.

Examples of the oxygen-containing gas may include air. Examples of theoxygen-containing compound may include ozone. Examples of the oxygengenerator may include peracetic acid and hydrogen peroxide. These oxygensources may be used alone or in combination.

Further, the oxygen concentration in a process stream selected from thegroup consisting of a gaseous stream as a gaseous phase and a liquidstream as a liquid phase may be, for example, relative to 1 mol of thetotal amount of hydrogen iodide and methyl iodide, about 0.25 mol orless (e.g., about 0.2 mol or less), preferably about 0.1 mol or less(e.g., about 0.05 mol or less), more preferably about 0.01 mol or less(e.g., about 1×10⁻³ mol or less), particularly about 1×10⁻⁴ mol or less(e.g., about 1×10⁻⁵ mol or less) or may be about 1×10⁻⁶ mol or less(e.g., about 1×10⁻⁷ mol or less).

Further, in at least one process stream selected from the groupconsisting of a stream of a process unit and a stream of a process line,the ratio of oxygen relative to carbon monoxide (O₂/CO) in each of agaseous phase and a liquid phase (for example, a gaseous phase) may be7% by volume or less (e.g., 5% by volume or less), for example, 2% byvolume or less (e.g., 1% by volume or less), preferably 0.5% by volumeor less (e.g., 0.1% by volume or less), more preferably 0.01% by volumeor less (e.g., 0.001% by volume or less), and particularly 0.0001% byvolume or less (e.g., 0.00001% by volume or less).

In the process stream, the concentration of oxygen in the liquid phaseis low practically, and the ratio (O₂/CO) of oxygen relative to carbonmonoxide may significantly fluctuate in some cases. The ratio (O₂/CO) ofoxygen relative to carbon monoxide in the liquid phase may be, forexample, 1000% by weight or less (10 times or less) (e.g., 500% byweight or less), or may be 250% by weight or less (e.g., 100% by weightor less), preferably 75% by weight or less (e.g., 50% by weight orless), more preferably 20% by weight or less (e.g., 10% by weight orless), or may be 5% by weight or less (e.g., 1% by weight or less),preferably 0.1% by weight or less (e.g., 0.01% by weight or less), morepreferably 0.001% by weight or less (e.g., 0.0001% by weight or less),and particularly 0.00005% by weight or less (e.g., 0.00001% by weight orless).

As described later, the volume percent (% by volume) and the weightpercent (% by weight) of each component described above may becalculated or converted mutually using an average molecular weight (aweighted average molecular weight).

Hereinafter, the present invention will be explained in detail withreference to the drawings if necessary. Each step and a main apparatusor unit for the corresponding step may be indicated by the samereference numeral or sign.

The process (or apparatus) for continuously producing acetic acid asshown in FIG. 1 to FIG. 4 comprises (1) a reaction step (a reactionsystem or a reactor) for carrying out a carbonylation reaction ofmethanol; (2) a step (a flash evaporation step or a flasher) forseparating the reaction mixture into a volatile phase (2A) and aless-volatile phase (2B); and (3) a step (a first distillation step, asplitter column) for distilling and separating the volatile phase (2A)to forma first overhead (3A), an acetic acid stream (3B), and a bottomliquid stream (higher boiling point fraction (or component)) (3C), thefirst overhead (3A) being rich in at least one lower boiling componentselected from the group consisting of methyl iodide and acetaldehyde,the acetic acid stream (3B) being rich in acetic acid; and furthercomprises (4) a purification section or purification step group forobtaining purified acetic acid from the acetic acid stream (3B) (steps(5) to (8)); (9) a separation section or separation step group forseparating at least acetaldehyde from the first overhead (3A) (steps(10) to (14)); and (15) an off-gas treatment section or off-gastreatment step group for absorption-treating an off-gas from the processwith an absorption solvent to separate the off-gas into a stream rich incarbon monoxide and a stream rich in acetic acid, methyl iodide, andmethyl acetate (steps (16) to (18)).

According to an embodiment of the present invention, the processcomprises, in addition to the above steps (1) to (3), at least onesection (step group or unit group) selected from the group consisting ofthe sections (4), (9), and (15). For example, the off-gas treatmentsection (15) is not necessarily needed. In the off-gas treatment section(15), treatment of off-gases from all units or lines is not necessarilyneeded, and off-gas(s) from predetermined process unit(s) or line(s) maybe treated. The gaseous phase or gaseous phase portion of the processhaving the predetermined oxygen concentration means all gaseous phasesof the process; the gas forming the gaseous phase includes an off-gasfrom the process irrespective of discharge to the outside of the system.The off-gas may be an “off-gas” to be subjected to the off-gas treatmentsection or may be an off-gas from a process unit and/or line.Hereinafter, each step will be explained in detail.

(1) Reaction Step (Reactor)

In the reaction step (1), methanol from a feed line 2 and carbonmonoxide from a feed line 4 are continuously fed to a reactor (1) in thepresence of a reaction medium containing a carbonylation catalyst systemand water to produce acetic acid by carbonylation of methanol. Thecarbon monoxide from the line 4 is mixed with hydrogen fed from a line 6for the purpose of increasing a catalytic activity, and the mixture isfed as a mixed gas 7 to the reactor (1). Methanol is fed to the reactor(1), and methanol is added to a distillation column of a seconddistillation step (5) via a line 3. Carbon monoxide from a line 5 ismixed with a less-volatile phase (a bottom catalyst liquid) from arecycle line 21 of an evaporator (2) in order to prevent precipitationof the catalyst and the mixed catalyst liquid is recycled to the reactor(1) via a line 22.

Moreover, in a liquid-liquid separation step (10), an upper phase 38 (anupper phase rich in acetic acid, methyl iodide, methyl acetate, andwater) and a lower phase 39 (a lower phase rich in methyl iodide andmethyl acetate) are biphasically formed in the decanter S2, and aportion 41 of the upper phase 38 and a portion 40 (or a first portion)of the lower phase 39 may be recycled to the reactor (1). A portion (ora second portion) 40 of the lower phase 39 may be subjected to an alkaneseparation step (distillation step) (14). Moreover, a portion 54 of acondensate (a portion of a condensate rich in acetic acid) of a secondoverhead 51 from a distillation column of a second distillation step (5)may also be mixed or merged with the less-volatile phase (2B) (the line21), and the mixture may be recycled to the reactor (1).

Fresh methanol may be fed to the reaction system (1) directly orindirectly, or methanol or a derivative thereof withdrawn from varioussucceeding distillation steps may be fed to the reaction system byrecycling to the reaction step. As such a raw material methanol, it ispreferred to use methanol from which oxygen has been removed beforehand.The carbon monoxide may be used as a pure gas or may be used as a gasdiluted with an inactive or inert gas (for example, nitrogen, helium,and carbon dioxide). If necessary, a waste gas containing carbonmonoxide obtained from the succeeding step(s) may be recycled to thereaction system. As such carbon monoxide or waste gas, it is preferredto use carbon monoxide or waste gas from which oxygen has been removedbeforehand.

In the following Tables, O₂ represents oxygen, H₂ denotes hydrogen, COrepresents carbon monoxide, CO₂ denotes carbon dioxide, CH₄ representsmethane N₂ denotes nitrogen, AD represents acetaldehyde, MeOH denotesmethanol, MeI represents methyl iodide, MA denotes methyl acetate, H₂Orepresents water, AcOH denotes acetic acid, HI represents hydrogeniodide, LiI denotes lithium iodide, FrOH represents formic acid, PrOHdenotes propionic acid, DME represents dimethyl ether, AcA denotesacetic anhydride, (CH₃)₂C═O represents acetone, EtOH denotes ethanol, EArepresents ethyl acetate, EtI denotes ethyl iodide, TOI represents totaliodine compounds, and HexI denotes hexyl iodide (the same applieshereinafter). Moreover, the concentration of HI represents aconcentration in terms of iodide ion. Metals are represented by atomicsymbols.

For reference, some of the following Tables describe an averagemolecular weight of a stream. The average molecular weight means aweighted average value calculated based on the molecular weight andproportion of each component contained in a stream. Incidentally, when agaseous-phase mixture has an average molecular weight (a weightedaverage molecular weight) A and contains a component having a molecularweight B, the volume percent (% by volume) or weight percent (% byweight) of the component can be calculated on the basis of the weightpercent (% by weight) or volume percent (% by volume) of the component.For a concentration of oxygen as one example, when the average molecularweight A of a gaseous-phase mixture is 62.2 (weighted average molecularweight) and the measured value of a volume percent D of oxygen in thegaseous-phase mixture is 7.0% by volume, a weight percent C of oxygencan be, for example, calculated from the molecular weight B (=32) ofoxygen and the following equation: (C(×100)×A)/B=D(×100), as follows:(C×62.2)/32=7, the weight percent C of oxygen=3.6% by weight. Thus, theweight percent (% by weight) and volume percent (% by volume) of eachcomponent in the gaseous-phase mixture can be calculated from the aboveequation. Accordingly, the following Tables denote componentconcentrations in only weight percent (% by weight).

For a sample (a gaseous phase and a liquid phase) in which a gaseouscomponent other than oxygen has a concentration less than the limit ofdetection (or detection limit), the concentration of the gaseouscomponent in the sample may be determined by forming a concentrate inthe same manner as in the case of the concentration of oxygen, measuringthe concentration of the gaseous component in the concentrate, andconverting the measured value into a concentration of the gaseouscomponent in the sample.

Moreover, for a sample (a gaseous phase and a liquid phase) in which acomponent has a concentration less than the detection limit (e.g., lessthan 0.1 ppb for a metal component, less than 1 ppm for organic matter),the concentration of the component in the sample may be determined byforming a concentrate of the component, measuring the concentration ofthe component in the concentrate, and converting the measured value intoa concentration of the component in the sample. In a case where thecomponent concentration in a sample is unmeasurable, the componentconcentration may be estimated according to distillation calculation andentrainment by evaporation. For example, with respect to amounts ofentrainment in adjacent stages or plates in an evaporation operation oran operation of a distillation column, the amount of entrainment in ahigher stage or plate corresponds to about 1 to 20% by weight that in alower stage or plate, and the concentration of a metal in a liquid in ahigher stage or plate is about 1 to 20% by weight of that in a lowerstage or plate. Based on such an estimated value, the concentration ofthe metal may be determined.

Incidentally, an inactive gas may be introduced in the process. Forexample, an inactive gas (such as nitrogen gas N₂) is fed to a processunit (such as a distillation column) for the purpose of regulating aninternal pressure of the unit, and/or an inactive gas (such as nitrogengas N₂) purge to a measuring instrument (such as a pressure gauge, athermometer, or a level gauge) is performed for the purpose ofpreventing an organic matter vapor from entering the measuringinstrument. Moreover, an inactive gas such as carbon monoxide gas CO maybe introduced instead of nitrogen gas N₂. Further, water, an alkalimetal compound, a methanol source, or others may be introduced to theprocess unit and line. In such a case, a concentration of a feedcomponent [for example, a composition of a gas (e.g., an inactive gassuch as nitrogen gas N₂ or carbon monoxide)] in the following Tables,which show a composition in the process unit and line, drasticallyincreases or changes depending on the feed component (such as aninactive gas) and an amount thereof.

For example, the raw material methanol (the line 2) may have thefollowing composition (unit: % by weight).

TABLE 1 Range Preferred range More preferred range O₂ 10% or less (e.g.,0.1 ppb to 10%), less than 1% (e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppbto 3.6% (e.g., 1 ppb ppt to 1000 ppm), e.g., 100 ppt to 100 to 2%) e.g.,less than 700 ppm ppm (e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 pptto 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO 0 to 1% (e.g., 1 ppt to0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO₂ 0 to 1% (e.g., 1 ppt to0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CH₄ 0 to 1% (e.g., 1 ppt to0.1%) 10 ppt to 0.01% 100 ppt to 0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.1%)10 ppt to 0.01% 100 ppt to 0.001% AD 0 to 1% (e.g., 1 ppb to 0.1%) 10ppb to 0.01% 100 ppb to 0.001% MeOH 95 to 100% 98 to 99.999% 99 to99.99% MeI 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% MA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.001% H₂O 1 ppm to 0.1% 10 ppm to 0.05% 100 ppm to 0.01% AcOH 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% HI 0 to 1%(e.g., 0.1 ppb to 0.5%) 1 ppb to 0.1% 10 ppb to 100 ppm FrOH 0 to 1%(e.g., 1 ppm to 0.1%) 5 to 100 ppm 10 to 30 ppm PrOH 0 to 1% (e.g., 0.1ppm to 0.1%) 1 to 100 ppm 10 to 30 ppm DME 0 to 1% (e.g., 1 ppb to 100ppm) 10 ppb to 30 ppm 100 ppb to 5 ppm AcA 0 to 1% (e.g., 1 ppb to 0.1%)10 ppb to 100 ppm 100 ppb to 50 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 50 ppm 100 ppb to 30 ppm EtOH 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm EA 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm EtI 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm Li 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb Fe 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm Ni 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm Cr 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm Mo 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm Zn 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm Cu 0 to 0.1% (e.g., 1 ppt to1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm

For example, the raw material carbon monoxide (the lines 4, 5) may havethe following composition (unit: % by weight).

TABLE 2 Range Preferred range More preferred range O₂ 10% or less (e.g.,0.1 ppb to 10%), 2 ppb to 1% (e.g., 10 50 ppb to 500 ppm e.g., 0.2 ppbto 3.6% (e.g., 1 ppb ppb to 0.1%), or (e.g., 100 ppb to 100 to 2%), orless than 7% (e.g., 0.1 20 ppb to 5000 ppm ppm), or ppb to 5%), e.g., 1ppb to 3% (e.g., 50 ppb to 1000 ppm 10 ppb to 1%) H₂ 0 to 1% (e.g., 1ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO 95 to 100% 98 to99.999% 99 to 99.99% CO₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01%100 ppt to 0.001% CH₄ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100ppt to 0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 pptto 0.001% AD 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.1 ppm MeOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.1 ppm MeI 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.1ppm MA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.1 ppmH₂O 0 to 1% (e.g., 1 ppm to 0.1%) 10 ppm to 0.05% 20 ppm to 0.01% AcOH 0to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.1 ppm FrOH 0 to1% (e.g., 1 ppm to 0.1%) 5 to 100 ppm 10 to 50 ppm PrOH 0 to 1% (e.g., 1ppb to 0.1%) 1 to 100 ppm 10 to 50 ppm DME 0 to 0.1% (e.g., 1 ppb to 100ppm) 10 ppb to 30 ppm 100 ppb to 0.1 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppbto 100 ppm) 10 ppb to 50 ppm 100 ppb to 0.1 ppm EtOH 0 to 1% (e.g., 1ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 0.1 ppm EA 0 to 1% (e.g., 1ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 0.1 ppm EtI 0 to 1% (e.g., 1ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 0.1 ppm

The composition of the mixed gas (the line 7) may, for example, besubstantially the same as (or similar to) the composition of the rawmaterial carbon monoxide (the lines 4, 5). Moreover, the composition ofthe mixed gas (the line 7) may have a component ratio (content of eachcomponent) obtained or calculated from the weighted average of thecomponent ratio of the raw material carbon monoxide (the line 4, 5) andthe component ratio of hydrogen (the line 6).

The carbonylation catalyst system usually contains a metal catalyst(such as a cobalt catalyst, a rhodium catalyst, or an iridium catalyst),a catalyst stabilizer or reaction accelerator, and/or a co-catalyst. Themetal catalysts may be used alone or in combination. The metal catalystmay preferably include a rhodium catalyst and an iridium catalyst (inparticular, a rhodium catalyst).

The metal catalyst may be used in the form of a simple metal, a metaloxide (including a complex metal oxide), a metal hydroxide, a metaliodide, a metal carboxylate (e.g., an acetate), a metal salt of aninorganic acid (e.g., a sulfate, a nitrate, and a phosphate), or a metalcomplex. It is preferred to use the metal catalyst in a form (e.g., acomplex form) dissolvable in a liquid phase (or a reaction medium orliquid). The rhodium catalyst may preferably include, for example, arhodium iodide complex {e.g., RhI₃, [RhI₂(CO)₄]⁻, and [Rh(CO)₂I₂]⁻} anda rhodium carbonyl complex.

The catalyst stabilizer or reaction accelerator may include an ionicmetal iodide capable of producing an iodide ion in the reaction medium,for example, an alkali metal iodide (e.g., lithium iodide, sodiumiodide, and potassium iodide). Among these stabilizers, lithium iodideis preferred. The catalyst stabilizer or reaction accelerator mayinclude, in addition to lithium iodide and analogous compounds, a metalcompound (such as a metal iodide or a complex) having a transition metal(including a metal or a corroded metal present in the reaction system,as shown in the following Tables). Examples of the transition metal mayinclude a group 6 element of the Periodic Table (such as molybdenum,chromium, or tungsten), a group 7 element of the Periodic Table (such asmanganese or rhenium), a group 8 element of the Periodic Table (such asiron, ruthenium, or osmium), a group 9 element of the Periodic Table(such as cobalt, rhodium, or iridium), a group 10 element of thePeriodic Table (such as nickel), a group 11 element of the PeriodicTable (such as copper), a group 12 element of the Periodic Table (suchas cadmium or zinc), and a group 13 element of the Periodic Table (suchas gallium or indium). These catalyst stabilizers or accelerators may beused alone or in combination according to the species of the metalcatalyst. For an iridium catalyst system, an alkali iodide metal is notnecessarily needed. As the co-catalyst, methyl iodide may be used.

A preferred carbonylation catalyst system may comprise a rhodiumcatalyst, a metal iodide as a catalyst stabilizer (e.g., lithiumiodide), and methyl iodide as a co-catalyst. To the reactor may be fed acatalyst mixture (a catalyst liquid) containing the carbonylationcatalyst system, and water. It is preferred that oxygen have beenremoved beforehand from such a catalyst mixture and water by heating orboiling.

The carbon monoxide partial pressure in the reactor may be a pressureof, for example, about 2 to 30 atmospheres and preferably about 4 to 15atmospheres. The carbonylation reaction produces hydrogen by a reactionof carbon monoxide with water. Hydrogen increases the catalyst activity.Thus hydrogen may be fed to the reactor if necessary. Hydrogen may befed to the reactor by recycling gaseous component(s) (includinghydrogen, carbon monoxide, or other gases) exhausted in the succeedingstep(s), if necessary after purifying the gaseous component(s). As suchhydrogen, it is preferred to use hydrogen having less oxygenconcentration. The hydrogen partial pressure in the reaction system maybe a pressure of, for example, about 0.5 to 250 kPa (e.g., about 1 to200 kPa), preferably about 5 to 150 kPa, and more preferably about 10 to100 kPa (e.g., about 10 to 50 kPa) in terms of absolute pressure.

The temperature of the carbonylation reaction may be, for example, about150 to 250° C., preferably about 160 to 230° C., and more preferablyabout 170 to 220° C. The reaction pressure (total reactor pressure),including partial pressures of by-products, may be, for example, about15 to 40 atmospheres. The space time yield of acetic acid in thereaction system may be, for example, about 5 to 50 mol/Lh, preferablyabout 8 to 40 mol/Lh, and more preferably about 10 to 30 mol/Lh.

In the reactor, the carbonylation reaction of methanol proceeds withforming an equilibrium between a liquid-phase reaction system and agaseous-phase system. The liquid-phase reaction system contains thereactant (s) and the metal catalyst component, and the gaseous-phasesystem comprises carbon monoxide, reaction by-products (hydrogen,methane, and carbon dioxide), and vaporized lower boiling components(e.g., methyl iodide, a product acetic acid, and methyl acetate).

The metal catalyst in the liquid phase has a concentration of, forexample, about 100 to 5000 ppm by weight, preferably about 200 to 3000ppm by weight, more preferably about 300 to 2000 ppm by weight, andparticularly about 500 to 1500 ppm by weight in the whole liquid phasein the reactor. The catalyst stabilizer or reaction accelerator in thewhole liquid phase in the reactor has a concentration of, for example,about 1 to 25% by weight, preferably about 2 to 22% by weight, and morepreferably about 3 to 20% by weight. The iodide ion in the reactionsystem may have a concentration of, for example, about 0.05 to 2.5 mol/Land preferably about 0.25 to 1.5 mol/L. The methyl iodide in the wholeliquid phase in the reactor has a concentration of, for example, about 1to 30% by weight, preferably about 5 to 25% by weight, and morepreferably about 6 to 20% by weight (e.g., about 8 to 18% by weight).

The reaction medium (or liquid phase) usually contains the productacetic acid, methyl acetate formed by a reaction of the product aceticacid and raw material methanol, and water. The acetic acid also plays asa solvent. Moreover, the reaction medium (or the liquid phase) usuallycontains unreacted raw material methanol. The proportion of methylacetate in the whole reaction liquid may be about 0.1 to 30% by weight,preferably about 0.3 to 20% by weight, and more preferably about 0.5 to10% by weight (e.g., about 0.5 to 6% by weight). The water in thereaction medium may have a low concentration, and may have, in the wholereaction liquid, a concentration of, for example, about 0.1 to 15% byweight, preferably about 0.5 to 10% by weight, and more preferably about0.8 to 5% by weight (e.g., about 1 to 3% by weight) and may usually beabout 1 to 10% by weight (e.g., about 2 to 5% by weight).

The reaction mixture (the crude reaction liquid) also contains variousby-products including acetaldehyde and by-products derived fromacetaldehyde. The present invention allows effective removal ofacetaldehyde in the separation section (9). Thus, the present inventionenables the concentration of acetaldehyde in the reactor to be decreasedand also enables the production of by-products derived from acetaldehydeto be prevented, although the reaction is a continuous reaction. Anacetaldehyde concentration in the liquid phase in the reactor may be,for example, not more than 1500 ppm by weight, e.g., about 10 to 1000ppm by weight, preferably about 50 to 500 ppm by weight, and morepreferably about 100 to 400 ppm by weight.

Examples of the by-products derived from acetaldehyde (acetaldehydederivatives) may include an aldehyde such as butyraldehyde,crotonaldehyde, 2-ethylcrotonaldehyde, and 2-ethylbutyraldehyde; aketone such as acetone or methyl ethyl ketone; an aldol condensationproduct thereof; and a C₂₋₁₂alkyl iodide such as ethyl iodide, propyliodide, butyl iodide, pentyl iodide, or hexyl iodide. The by-productsmay also include formic acid, a carboxylic acid having 3 or more carbonatoms [e.g., a straight chain or branched chain carboxylic acid such aspropionic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoicacid, octanoic acid, and a higher fatty acid having 9 or more carbonatoms (e.g., a C₃₋₁₂alkanecarboxylic acid)]; an alkyl alcohol (e.g.,ethanol, butyl alcohol, 2-ethylbutyl alcohol, hexyl alcohol, heptylalcohol, octyl alcohol, and an alkyl alcohol having 9 or more carbonatoms (e.g., a C₃₋₁₂alkyl alcohol); and a hydrocarbon having 2 or morecarbon atoms (e.g., a C₂₋₁₂alkane). In the liquid phase, an ester ofmethanol or the above alkyl alcohol with acetic acid or the abovecarboxylic acid (such as methyl acetate) and a dialkyl ether such asdimethyl ether are also produced secondarily. A total concentration ofthese by-products may be about 0.1 ppb to 100 ppm (e.g., about 0.5 ppbto 50 ppm) and preferably about 1 ppb to 10 ppm (e.g., about 2 ppb to 1ppm) in the whole process including the liquid phase system. Thus, theconcentration of these by-products may be omitted from the descriptionor indication in each of the following steps.

The alkyl iodide having 2 or more carbon atoms (such as hexyl iodide)may have a concentration of, for example, about 0.1 ppb to 1 ppm (e.g.,about 0.5 to 500 ppb) and preferably about 1 to 100 ppb. Thealkanecarboxylic acid (such as formic acid or propionic acid) may have aconcentration of, for example, about 0.1 to 500 ppm (e.g., about 1 to500 ppm) and preferably about 3 to 100 ppm.

The dimethyl ether (DME) may have a concentration of not more than 0.5%by weight (e.g., about 0.1 to 1000 ppm), preferably about 1 to 500 ppm(e.g., about 2 to 300 ppm), and more preferably about 3 to 200 ppm(e.g., about 5 to 100 ppm).

Furthermore, 3-hydroxyalkanal (such as 3-hydroxybutanal) is alsoproduced secondarily. The 3-hydroxyalkanal content of the liquid phasemay be about 100 ppm or less (e.g., about 0.1 ppb to 100 ppm) andpreferably about 0.5 ppb to 50 ppm. These by-products are usuallyincreased in proportion to the square to the cube of the concentrationof acetaldehyde.

Moreover, acetaldehyde and the by-products derived from acetaldehyde(for example, other aldehydes, the ketone, and the aldol condensationproduct) belong to permanganate reducing compounds (PRC's). Thus, it ispreferred to separate and remove acetaldehyde, which is a mainby-product, from the reaction mixture and to recover useful components(e.g., methyl iodide) from the process stream(s) for effectiveutilization. Incidentally, although the C₂₋₁₂alkyl iodide, includingmethyl iodide, also belongs to the PRC's, methyl iodide is excluded fromthe PRC's in this description and claims.

The reaction system (the liquid phase) also contains metals produced bycorrosion, for example, iron, nickel, chromium, molybdenum, cobalt, andzirconium. The reaction system (the liquid phase) may contain not morethan 2000 ppm (e.g., about 1 to 1000 ppm) each of these corroded metals.The total corroded metal content may be about not more than 10000 ppm(e.g., about 5 to 5000 ppm). Incidentally, a liquid stream in thedownstream side of the reaction system (the liquid phase) may contain acorroded metal in the same proportion as the above. Thus, theconcentration of the corroded metals in the liquid stream is omittedfrom the description or indication in each of the following steps.

As described above, the reaction mixture (the crude reaction liquid)contains acetic acid, lower boiling components or impurities, eachhaving a boiling point lower than acetic acid (e.g., methyl iodide,methyl acetate, water, and acetaldehyde), and higher boiling componentsor impurities, each having a boiling point higher than acetic acid[e.g., a metal catalyst component (e.g., a rhodium catalyst), lithiumiodide as a catalyst stabilizer, and a C₃₋₁₂alkanecarboxylic acid (e.g.,propionic acid)]. Thus, in the process for producing acetic acid, apurified acetic acid is produced by removing impurities from thereaction mixture (the crude reaction liquid).

The reaction system is an exothermic reaction system that accompaniesheat generation, and the reaction temperature may be controlled (orregulated) by recycling of the condensate which has been cooled or fromwhich heat has been removed, installation of a heat-removable (orheat-removing) unit or a cooling unit (e.g., a jacket), or other means.In order to remove part of the reaction heat, a vapor (vent gas) fromthe reactor may be cooled in a condenser, a heat exchanger, or othermeans to separate the vapor into liquid components and gaseouscomponents, and the liquid components and/or the gaseous components maybe recycled to the reactor.

A gaseous phase (a line 8) from the reactor (1) is cooled and condensedin a condenser to form a condensate 10 and a noncondensable gas 9containing carbon monoxide in relatively large quantity. The condensate10 is returned to the reactor (1), and the noncondensable gas 9 isintroduced into a gas-liquid separating pot or buffer tank S1. Anoncondensable gas (an off-gas rich in carbon monoxide and methyliodide) 11 from the tank is treated in an off-gas treatment section (15)[for example, a high-pressure absorption column (16)]. Typically, atleast a portion of the noncondensable gas (off-gas) 11 containing carbonmonoxide is treated in the off-gas treatment section (15). A portion ofthe noncondensable gas (off-gas) can partly be introduced into theflasher or evaporator (2) via a line 172 (or introduced into a liquidphase or a volatile phase (gas) in the flasher) to stabilize thecatalyst in the flasher or evaporator (2) [or to prevent precipitationof the metal catalyst (e.g., a rhodium catalyst)].

For example, the gaseous phase (gaseous phase portion or gaseous stream)mixture (the line 8) from the reactor may have the followingcomposition.

When a gaseous-phase mixture has an average molecular weight (a weightedaverage molecular weight) A and contains a component having a molecularweight B, the volume percent (% by volume) or weight percent (% byweight) of the component can be calculated on the basis of the weightpercent (% by weight) or volume percent (% by volume) of the component.For a concentration of oxygen as one example, when the average molecularweight A of a gaseous-phase mixture is 62.2 (weighted average molecularweight) and the measured value of a volume percent D of oxygen in thegaseous-phase mixture is 7.0% by volume, a weight percent C of oxygencan be, for example, calculated from the molecular weight B (=32) ofoxygen and the following equation: (C(×100)×A)/B=D(×100), as follows:(C×62.2)/32=7, the weight percent C of oxygen=3.6% by weight. Thus, theweight percent (% by weight) and volume percent (% by volume) of eachcomponent in the gaseous-phase mixture can be calculated from the aboveequation. Accordingly, the following Tables denote componentconcentrations in only weight percent (% by weight).

TABLE 3 Average molecular weight 62.62 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 20ppb to 0.3% 50 ppb to 0.1% (e.g., ppt to 5%), e.g., less than 3.6% 100ppb to 200 ppm) (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppbto 0.5%) H₂ 0 to 5% (e.g., 0.01 to 5%) 20 ppb to 2% 100 ppb to 0.1%(e.g., 30 ppb to 1%) (e.g., 1 to 100 ppm) CO 0.1 to 70% (e.g., 1 to 50%)3 to 30% 7 to 20% CO₂ 0 to 5% (e.g., 0.01 to 5%) 0.05 to 2% 0.1 to 1%CH₄ 0 to 5% (e.g., 0.01 to 5%) 0.05 to 3% 0.1 to 2% N₂ 0 to 5% (e.g.,0.01 to 5%) 0.05 to 3% 0.1 to 2% AD 0.001 to 5% 0.01 to 2% 0.02 to 1%MeOH 0.1 ppm to 1% 1 ppm to 0.5% 10 ppm to 0.1% MeI 1 to 95% (e.g., 5 to90%) 10 to 80% 20 to 70% MA 0.1 to 15% 0.5 to 10% 1 to 7% H₂O 0.1 to 15%0.5 to 10% 1 to 7% AcOH 1 to 50% 2 to 40% 5 to 30% FrOH 0 to 1% (e.g.,0.01 ppm to 1%) 0.1 ppm to 0.5% 1 ppm to 0.1% PrOH 0 to 1% (e.g., 0.01ppm to 1%) 0.01 ppm to 0.5% 0.1 ppm to 0.1% DME 0 to 1% (e.g., 0.1 ppmto 1%) 1 ppm to 0.5% 10 ppm to 0.2% (CH₃)₂C═O 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.5% 10 ppm to 0.2% EtOH 0 to 1% (e.g., 0.1 ppm to 1%) 1ppm to 0.5% 10 ppm to 0.2% EA 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to0.5% 10 ppm to 0.2% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10ppm to 0.2% LiI 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 100ppt to 1 ppm Rh 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 100ppt to 1 ppm Fe 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 100ppt to 1 ppm Ni 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm Cr 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm Mo 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm Zn 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm Cu 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm

For example, the condensate 10 from the condenser may have the followingcomposition.

TABLE 4 Average molecular weight 88.44 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, 10%), e.g., 0.2 ppb to 3.6% (e.g., ppt to 1000 ppm),e.g., 100 ppt to 100 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppmto 0.01% CO 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.5% CO₂0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.2% CH₄ 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% N₂ 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% AD 0 to 1% (e.g., 1 ppmto 1%) 10 ppm to 0.5% 50 ppm to 0.2% MeOH 0 to 1% (e.g., 1 ppm to 1%) 10ppm to 0.5% 50 ppm to 0.2% MeI 1 to 95% 5 to 90% 10 to 80% MA 0.1 to 20%0.5 to 10% 1 to 5% H₂O 0.1 to 20% 0.5 to 10% 1 to 7% AcOH 1 to 50% 3 to40% 5 to 30% FrOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.1% 10 ppm to0.01% PrOH 0 to 1% (e.g., 0.1 ppm to 1%) 0.5 ppm to 0.1% 1 ppm to 0.01%DME 0 to 2% (e.g., 1 ppm to 2%) 5 ppm to 0.5% 10 ppm to 0.1% (CH₃)₂C═O 0to 2% (e.g., 1 ppm to 2%) 5 ppm to 0.5% 10 ppm to 0.1% EtOH 0 to 2%(e.g., 1 ppm to 2%) 5 ppm to 0.5% 10 ppm to 0.1% EA 0 to 2% (e.g., 1 ppmto 2%) 5 ppm to 0.5% 10 ppm to 0.1% EtI 0 to 2% (e.g., 1 ppm to 2%) 5ppm to 0.5% 10 ppm to 0.1% LiI 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 pptto 10 ppm 100 ppt to 1 ppm Rh 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 pptto 10 ppm 100 ppt to 1 ppm Fe 0 to 0.1% (e.g., 1 ppt to 100 ppm) 10 pptto 10 ppm 100 ppt to 1 ppm Ni 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 pptto 1 ppm 100 ppt to 0.1 ppm Cr 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 pptto 1 ppm 100 ppt to 0.1 ppm Mo 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 pptto 1 ppm 100 ppt to 0.1 ppm Zn 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 pptto 1 ppm 100 ppt to 0.1 ppm Cu 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 pptto 1 ppm 100 ppt to 0.1 ppm

For example, the noncondensable gas 9 from the condenser may have thefollowing composition.

The composition of the noncondensable gas (off-gas) 11 from the tank S1may be substantially the same as (or similar to) the composition of thenoncondensable gas 9 from the condenser.

TABLE 5 Average molecular weight 29.86 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% (e.g., 500ppb to 500 ppm 10%), e.g., 10 ppb to 3.6% (e.g., 100 ppb to 0.1%), or(e.g., 1 to 100 ppm), or 20 ppb to 2%), or less than 7% 20 ppb to 0.3%50 ppb to 0.1% (e.g., (e.g., 1 ppt to 5%), e.g., less 100 ppb to 200ppm) than 3.6% (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 10% (e.g., 0.1 ppm to 10%) 1 ppm to 5% 10 ppm to 2% CO 1to 99% 5 to 90% 10 to 85% CO₂ 0.01 to 5% 0.1 to 3% 0.2 to 2% CH₄ 0.1 to15% 0.5 to 10% 1 to 6% N₂ 0.1 to 20% 0.5 to 15% 1 to 10% AD 0 to 1%(e.g., 0.001 to 1%) 0.01 to 0.5% 0.02 to 0.2% MeOH 0 to 1% (e.g., 1 ppmto 1%) 5 ppm to 0.5% 10 ppm to 0.1% MeI 1 to 90% (e.g., 5 to 80%) 10 to70% 20 to 50% MA 0 to 5% (e.g., 0.001 to 5%) 0.01 to 1% 0.05 to 0.5% H₂O0 to 5% (e.g., 0.001 to 5%) 0.01 to 1% 0.05 to 0.5% AcOH 0 to 5% (e.g.,0.001 to 5%) 0.01 to 1% 0.05 to 0.5% FrOH 0 to 1% (e.g., 0.1 ppm to0.5%) 1 ppm to 0.2% 10 ppm to 0.1% PrOH 0 to 1% (e.g., 0.1 ppm to 0.5%)0.5 ppm to 0.2% 1 ppm to 0.1% DME 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppmto 0.2% 10 ppm to 0.1% (CH₃)₂C═O 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to0.1% 5 ppm to 0.05% EtOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5ppm to 0.05% EA 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to0.05% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% LiI0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100 ppt to 0.1 ppm Rh0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100 ppt to 0.1 ppm Fe0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100 ppt to 0.1 ppm Ni0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100 ppt to 0.01 ppmCr 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100 ppt to 0.01ppm Mo 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100 ppt to0.01 ppm Zn 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100 pptto 0.01 ppm Cu 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100ppt to 0.01 ppm

The reaction mixture 12 from the reactor (1) is introduced or fed to theflasher (evaporator) (2) via a line 12 for flash evaporation to form avolatile phase (2A) and a less-volatile phase (2B); the volatile phase(2A) contains the product acetic acid, methyl iodide, acetaldehyde,methyl acetate, water, or other components, and the less-volatile phase(2B) contains the rhodium catalyst and lithium iodide.

For example, the reaction mixture 12 may have the following composition.

TABLE 6 Average molecular weight 65.23 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 1ppt 10 ppt to 300 ppm, e.g., 10%), e.g., 0.2 ppb to 3.6% to 1000 ppm),e.g., less 100 ppt to 100 ppm (e.g., 1 ppb to 2%) than 700 ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 100 ppmto 0.1% (e.g., 10 ppm to 0.2%) CO 0.1 ppm to 5% 10 ppm to 1% 100 ppm to0.5% (e.g., 1 ppm to 2%) CO₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5%100 ppm to 0.1% (e.g., 10 ppm to 0.2%) CH₄ 0 to 1% (e.g., 0.1 ppm to 1%)1 ppm to 0.5% 100 ppm to 0.1% (e.g., 10 ppm to 0.2%) N₂ 0 to 1% (e.g.,0.1 ppm to 1%) 1 ppm to 0.5% 100 ppm to 0.1% (e.g., 10 ppm to 0.2%) AD 0to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 100 ppm to 0.1% (e.g., 10 ppmto 0.2%) MeOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to0.25% (e.g., 10 ppm to 0.2%) (e.g., 50 ppm to 0.2%), e.g., 100 ppm to0.1% MeI 1 to 20% (e.g., 2 to 17%) 4 to 15% 5 to 13% MA 0.5 to 7% (e.g.,1 to 4%) 1.5 to 5% 1.7 to 4% (e.g., 1.5 to 3%) (e.g., 1.8 to 2.5%) H₂O0.1 to 12% (e.g., 0.5 to 5%) 0.8 to 3% 1 to 2.5% AcOH 30 to 95% (e.g.,40 to 90%) 50 to 85% 60 to 80% HI 0 to 1% (e.g., 0.001 to 1%), 0.003 to0.5% 0.005 to 0.3% (e.g., e.g., 0.002 to 0.8% (e.g., 0.02 to 0.5%) 0.05to 0.3%), (e.g., 0.01 to 0.7%) e.g., 0.05 to 0.2% FrOH 0 to 1% (e.g., 1ppm to 0.1%) 5 to 500 ppm 10 to 200 ppm PrOH 0 to 1% (e.g., 1 ppm to0.2%) 5 ppm to 0.1% 30 to 300 ppm (e.g., 10 to 500 ppm) DME 0 to 1%(e.g., 0.1 ppm to 0.5%) 1 ppm to 0.3% 10 to 500 ppm (e.g., 5 ppm to0.1%) (e.g., 10 to 300 ppm), e.g., 10 to 100 ppm (CH₃)₂C═O 0 to 1%(e.g., 1 ppm to 0.1%) 10 to 300 ppm 20 to 100 ppm EtOH 0 to 1% (e.g., 1ppm to 0.1%) 10 to 300 ppm 20 to 100 ppm EA 0 to 1% (e.g., 1 ppm to0.1%) 10 to 300 ppm 20 to 100 ppm EtI 0 to 1% (e.g., 1 ppm to 0.1%) 10to 300 ppm 20 to 100 ppm LiI 0.1 to 25% (e.g., 1 to 20%) 5 to 23% 7 to20% (e.g., 5 to 17%) (e.g., 8 to 15%) Rh 100 ppm to 0.5% 200 ppm to 0.2%500 to 1500 ppm Fe 0 to 1% (e.g., 10 ppm to 1%) 100 ppm to 0.7% 500 ppmto 0.5% Ni 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 100 ppm to 0.1%Cr 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 100 ppm to 0.1% Mo 0 to1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.05% Zn 0 to 1%(e.g., 10 ppm to 1%) 100 ppm to 0.7% 500 ppm to 0.5% Cu 0 to 1% (e.g.,0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 30 ppm

(2) Flash Evaporation Step

In the flash evaporation step (2), as described above, the reactionmixture is separated into the volatile phase (2A) and the less-volatilephase (2B), and the less-volatile phase or catalyst liquid (2B) isrecycled to the reactor of the reaction step (1) via the recycle line21.

For example, the less-volatile phase (2B) (the line 21) may have thefollowing composition.

TABLE 7 Average molecular weight 63.47 Range Preferred range Morepreferred range Metal catalyst 200 ppm to 0.5% 500 ppm to 0.4% 0.1 to0.3% Ionic iodide 1 to 60% (e.g., 2 to 50%) 3 to 40% (e.g., 5 to 35%) 5to 25% (e.g., , 8 to 20%) O₂ 10% or less (e.g., 0.1 ppb to 10%), lessthan 1% (e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppbppt to 1000 ppm), e.g., 100 ppt to 100 to 2%) e.g., less than 700 ppmppm (e.g., 1 ppt to 500 ppm) H₂ 0 to 0.1% (e.g., 1 ppt to 100 ppm) 30ppt to 2 ppm 50 ppt to 1 ppm CO 0 to 1% (e.g., 5000 ppt to 30 ppm) 300ppb to 20 ppm 500 ppb to 5 ppm (e.g., 1000 ppb to 10 ppm) (e.g., 200 ppbto 1 ppm) CO₂ 0 to 0.1% (e.g., 1 ppt to 100 ppm) 30 ppt to 2 ppm 50 pptto 1 ppm CH₄ 0 to 0.1% (e.g., 1 ppt to 100 ppm) 30 ppt to 2 ppm 50 pptto 1 ppm N₂ 0 to 0.1% (e.g., 1 ppt to 100 ppm) 30 ppt to 2 ppm 50 ppt to1 ppm AD 0 to 1500 ppm 5 to 700 ppm 10 to 400 ppm (e.g., 10 to 0.1%)(e.g., 30 to 500 ppm) (e.g., 20 to 200 ppm), e.g., 50 to 300 ppm MeOH 0to 1% (e.g., 0 to 0.8%) 0 to 0.3% 0 to 0.2% MeI 0.01 to 8% (e.g., 0.05to 6%) 0.1 to 4% (e.g., 0.5 to 2.5%) 0.3 to 2.5% (e.g., 0.5 to 2%) MA0.6 to 20% (e.g., 0.7 to 15%) 0.7 to 10% (e.g., 0.8 to 5%) 0.9 to 3%(e.g., 0.9 to 2%) H₂O 0.1 to 12% (e.g., 0.5 to 10%) 0.7 to 8% (e.g., 0.8to 5%) 0.8 to 3% (e.g., 0.8 to 2%) AcOH 35 to 95% (e.g., 45 to 90%) 60to 90% 50 to 85% HI 0.001 to 1% (e.g., 0.01 to 0.7%) 0.003 to 0.6%(e.g., 0.02 to 0.5%) 0.005 to 0.4% (e.g., 0.05 to 0.3%) FrOH 0 to 0.1%(e.g., 1 ppm to 0.1%) 5 to 500 ppm 10 to 200 ppm PrOH 0 to 0.2% (e.g., 1ppm to 0.2%) 5 ppm to 0.1% 30 to 300 ppm (e.g., 10 to 500 ppm) DME 0 to0.5% (e.g., 0.1 to 0.1%) 1 to 500 ppm 5 to 500 ppm (e.g., 3 to 200 ppm)(e.g., 10 to 300 ppm), e.g., 5 to 100 ppm (CH₃)₂C═O 0 to 0.1% (e.g.,0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EtOH 0 to 0.1% (e.g., 0.01ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EA 0 to 0.1% (e.g., 0.01 ppm to0.1%) 0.1 to 100 ppm 1 to 50 ppm EtI 0 to 0.1% (e.g., 0.01 ppm to 0.1%)0.1 to 100 ppm 1 to 50 ppm LiI 0.1 to 33% (e.g., 1 to 26%) 5 to 30%(e.g., 6 to 21%) 8 to 27% (e.g., 8 to 24%), e.g., 9 to 19% Rh 150 to7000 ppm 300 to 2500 ppm 600 to 1800 ppm Fe 0 to 1% (e.g., 10 ppm to 1%)100 ppm to 0.7% 500 ppm to 0.5% Ni 0 to 0.5% (e.g., 1 ppm to 0.5%) 10ppm to 0.2% 100 ppm to 0.1% Cr 0 to 0.5% (e.g., 1 ppm to 0.5%) 10 ppm to0.2% 100 ppm to 0.1% Mo 0 to 0.5% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2%50 ppm to 0.05% Zn 0 to 1% (e.g., 10 ppm to 1%) 100 ppm to 0.7% 500 ppmto 0.5% Cu 0 to 0.1% (e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 30 ppm

The less-volatile phase (2B) also contains metals produced by corrosion,for example, iron, nickel, chromium, molybdenum, cobalt, zirconium,zinc, and copper. The less-volatile phase (2B) may contain about notmore than 2000 ppm (e.g., about 1 to 1000 ppm) each of these corrodedmetals.

Further, the less-volatile phase or catalyst liquid (2B) is mixed withthe portion 54 of the condensate (the portion of the condensate rich inacetic acid) of the second overhead 51 from the distillation column(dehydration column) of the second distillation step (5) at the recycleline 21, and the mixture is recycled to the reactor (1) of the reactionstep (1).

For example, the condensate 54 may have the following composition.

TABLE 8 Average molecular weight 56.08 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb ppt to1000 ppm), e.g., 100 ppt to 100 to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppt to 1%) 0.1 ppb to0.1% 1 to 10 ppm (e.g., 1 ppb to 100 ppm) CO 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.5% 10 to 100 ppm (e.g., 2 ppm to 0.1%) CO₂ 0 to 1% (e.g.,0.1 ppt to 1%) 0.1 ppb to 0.1% 1 to 10 ppm (e.g., 1 ppb to 100 ppm) CH₄0 to 1% (e.g., 0.1 ppt to 1%) 0.1 ppb to 0.1% 1 to 10 ppm (e.g., 1 ppbto 100 ppm) N₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 to 100 ppm(e.g., 2 ppm to 0.1%) AD 10 ppm to 0.5% 50 ppm to 0.2% 100 ppm to 0.1%MeOH 0 to 2% (e.g., 10 ppm to 2%) 50 ppm to 1% 100 ppm to 0.5% MeI 1 to30% 2 to 20% 5 to 15% MA 1 to 20% 2 to 15% 3 to 12% H₂O 1 to 20% 2 to15% 3 to 10% AcOH 30 to 95% 50 to 90% 60 to 85% FrOH 0 to 1% (e.g., 0.1ppm to 1%) 10 ppm to 0.1% 30 to 500 ppm PrOH 0 to 1% (e.g., 0.1 ppm to0.1%) 1 to 100 ppm 5 to 50 ppm DME 0 to 2% (e.g., 0.1 ppm to 2%) 1 ppmto 1% 10 ppm to 0.2% (CH₃)₂C═O 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to0.5% 10 ppm to 0.1% EtOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10ppm to 0.1% EA 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to0.1% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1% LiI0 to 0.1% (e.g., 0.001 ppb to 10 ppm) 0.01 ppb to 1 ppm 0.1 ppb to 0.1ppm Rh 0 to 0.1% (e.g., 0.001 ppb to 10 ppm) 0.01 ppb to 1 ppm 0.1 ppbto 0.1 ppm Fe 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 5 ppm 1000ppt to 1 ppm Ni 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100ppt to 0.05 ppm Cr 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm100 ppt to 0.01 ppm Mo 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1ppm 100 ppt to 0.01 ppm Zn 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to0.1 ppm 100 ppt to 0.01 ppm Cu 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 pptto 0.1 ppm 100 ppt to 0.01 ppm

Unless otherwise specifically noted hereinafter, the concentrations ofiron Fe, nickel Ni, chromium Cr, molybdenum Mo, zinc Zn, and copper Cuin a condensate may be in the ranges as described in the above Table 8.

At least a portion of the volatile phase (2A) from the flasher(evaporator) (2) is fed to a distillation column (splitter column) ofthe first distillation step (3) via a feed line 23. A portion 24 of thevolatile phase from the flasher (evaporator) (2) is cooled and condensedin first and second condensers sequentially to form condensates 26, 28and noncondensable gases (off-gases) 25, 30. The condensates 26, 28 arerecycled to the reactor (1) via a hold tank T1 and a recycle line 27,thereby cooling the reaction system of the reactor (1).

A gaseous phase 29 from the hold tank T1 is cooled in the secondcondenser, and the noncondensable gas (off-gas) 30 from the secondcondenser is fed to the off-gas treatment section (15) (a low-pressureabsorption column (17)). Incidentally, as described later, anoncondensable gas 192 from the off-gas treatment section (15) is alsofed between the first condenser and the second condenser, and is cooledand condensed in the second condenser. To the hold tank T1 is also fed acondensate (a condensate rich in methyl iodide) 193 from the off-gastreatment section (15).

As described above, a portion 172 of an overhead stream 171 from theoff-gas treatment section (15) (the high-pressure absorption column(16)) is introduced to the flasher (catalyst separation column) (2).

The flash evaporation may include a thermostatic flash in which thereaction mixture is heated and depressurized, an adiabatic flash inwhich the reaction mixture is depressurized without heating, or acombination of these flash conditions. By such a flash evaporation, thereaction mixture may be separated into the vapor phase and the liquidphase. For example, the flash distillation (evaporation) may be carriedout at a temperature of about 100 to 250° C. (e.g., about 120 to 230°C.), preferably about 150 to 220° C. (e.g., about 160 to 210° C.), andmore preferably about 170 to 200° C. The flash distillation(evaporation) may be carried out at a pressure (absolute pressure) ofabout 0.03 to 1 MPa (e.g., about 0.05 to 1 MPa), preferably about 0.07to 0.7 MPa, and more preferably about 0.1 to 0.5 MPa (e.g., about 0.15to 0.4 MPa). Moreover, the less-volatile phase or catalyst liquid (2B)may have a temperature of, for example, about 80 to 200° C. (e.g., about90 to 180° C.), preferably about 100 to 170° C. (e.g., about 120 to 160°C.), and more preferably about 130 to 160° C. Under such a relativelyhigh temperature (and high pressure) condition, hydrogen iodide iseasily produced, and iodine is easily formed depending on theconcentration of oxygen. The present invention allows effectivelyreduced formation of iodine even if hydrogen iodide is produced.

For example, the volatile phase (2A) (lines 23, 24) may have thefollowing composition.

TABLE 9 Average molecular weight 70.17 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 ppt20 ppb to 0.3% 50 ppb to 0.1% (e.g., to 5%), e.g., less than 3.6% (e.g.,100 ppb to 200 ppm) 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% CO100 ppm to 3% 0.1 to 2% 0.2 to 1% CO₂ 10 ppm to 2% 100 ppm to 1% 0.02 to0.5% (e.g., 0.1 to 0.5%) CH₄ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5%100 ppm to 0.1% N₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to0.1% AD 0 to 1% (e.g., 0.01 to 0.5%) 0.02 to 0.2% (e.g., 0.03 to 0.15%)0.04 to 0.1% MeOH 0 to 2% (e.g., 10 ppm to 2%) 50 ppm to 1.5% 500 ppm to0.7% (e.g., 100 ppm to 1%) (e.g., 0.1% to 0.5%) MeI 10 to 60% 15 to 50%20 to 45% MA 1 to 15% (e.g., 2 to 12%) 4 to 10% 5 to 8% H₂O 0.1 to 10%0.8 to 8% 1.5 to 4% AcOH 20 to 80% (e.g., 30 to 75%) 40 to 70% (e.g., 50to 65%) 60 to 70% HI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppmto 0.1% (e.g., 5 ppm to 0.3%) LiI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to0.5% 30 ppm to 0.03% (e.g., 10 ppm to 0.1%) FrOH 0 to 1% (e.g., 0.1 ppmto 0.1%) 1 to 100 ppm 5 to 50 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1to 300 ppm 10 to 100 ppm (e.g., 5 to 200 ppm) DME 0 to 1% (e.g., 0.1 ppmto 0.1%) 1 to 700 ppm 5 to 500 ppm (e.g., 1 to 100 ppm) (e.g., 5 to 50ppm) (CH₃)₂C═O 0 to 1% (e.g., 1 ppm to 0.1%) 10 to 500 ppm 20 to 100 ppmEtOH 0 to 1% (e.g., 1 ppm to 0.1%) 10 to 500 ppm 20 to 100 ppm EA 0 to1% (e.g., 1 ppm to 0.1%) 10 to 500 ppm 20 to 100 ppm EtI 0 to 1% (e.g.,1 ppm to 0.1%) 10 to 500 ppm 20 to 100 ppm LiI 0 to 0.5% (e.g., 1 ppb to0.1%) 0.01 to 500 ppm 0.1 to 200 ppm Rh 0 to 0.5% (e.g., 1 ppb to 0.1%)0.01 to 500 ppm 0.1 to 100 ppm Fe 0 to 1% (e.g., 0.1 ppm to 0.2%) 1 ppmto 0.1% 10 to 500 ppm Ni 0 to 0.5% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm10 to 200 ppm Cr 0 to 0.5% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 10 to200 ppm Mo 0 to 0.5% (e.g., 0.01 to 500 ppm) 0.1 to 200 ppm 1 to 100 ppmZn 0 to 1% (e.g., 0.1 ppm to 0.2%) 1 ppm to 0.1% 10 to 500 ppm Cu 0 to0.1% (e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm

For example, the condensate 26 from the first condenser may have thefollowing composition.

The composition of the condensate (recycle line) 27 recycled from thehold tank T1 to the reactor (1) may be substantially the same as (orsimilar to) the composition of the condensate 26 from the firstcondenser. Moreover, the composition of the above condensate (recycleline) 27 may have a component ratio (content of each component) obtainedor calculated from the weighted average of the component ratio of thecondensate 26 from the first condenser and the component ratio of thecondensate 28 from the second condenser.

TABLE 10 Average molecular weight 70.60 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb 1 ppt to1000 ppm), e.g., 100 ppt to 100 to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.01% CO 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100ppm to 0.1% CO₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to0.1% CH₄ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% N₂ 0to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% AD 0 to 1%(e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% MeOH 0 to 1% (e.g., 1ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% MeI 1 to 95% 5 to 90% 10 to70% MA 0.1 to 40% 0.5 to 20% 1 to 10% H₂O 0.1 to 40% 0.5 to 20% 1 to 7%AcOH 1 to 95% 10 to 90% 30 to 80% HI 0 to 0.5% (e.g., 0.01 ppm to 0.1%)0.1 ppm to 0.01% 1 ppm to 0.001% LiI 0 to 0.5% (e.g., 0.01 ppm to 0.1%)0.1 ppm to 0.05% 1 ppm to 0.01% FrOH 0 to 0.5% (e.g., 0.01 ppm to 0.1%)0.1 ppm to 0.05% 1 ppm to 0.01% PrOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppmto 0.1% 2 ppm to 0.01% DME 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.01% (CH₃)₂C═O 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to0.1% 5 ppm to 0.05% EtOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5ppm to 0.05% EA 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to0.05% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% LiI0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm Rh0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm Fe0 to 0.5% (e.g., 0.1 ppm to 0.2%) 1 ppm to 0.1% 10 to 500 ppm Ni 0 to0.2% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 10 to 200 ppm Cr 0 to 0.2%(e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 10 to 200 ppm Mo 0 to 0.1% (e.g.,0.01 to 500 ppm) 0.1 to 200 ppm 1 to 100 ppm Zn 0 to 0.5% (e.g., 0.1 ppmto 0.2%) 1 ppm to 0.1% 10 to 500 ppm Cu 0 to 0.1% (e.g., 0.001 to 100ppm) 0.01 to 50 ppm 0.1 to 10 ppm

For example, the noncondensable gas (off-gas) 25 from the firstcondenser may have the following composition.

TABLE 11 Average molecular weight 46.42 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0.1 to 10% 0.2 to 5% 0.5 to 5% CO 1 to 99% 5 to 90% 10 to 80%CO₂ 0.1 to 20% 0.2 to 15% 0.5 to 8% CH₄ 0.1 to 20% 0.2 to 15% 0.5 to 8%N₂ 0.1 to 20% 0.2 to 15% 0.5 to 8% AD 0.001 to 3% 0.01 to 1% 0.02 to0.5% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to 0.1% MeI 1to 95% 10 to 90% 20 to 80% MA 0.1 to 20% 0.5 to 10% 1 to 5% H₂O 0.01 to2% 0.05 to 1% 0.1 to 0.5% AcOH 0.1 to 20% 0.5 to 10% 1 to 5% FrOH 0 to1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% PrOH 0 to 1%(e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% DME 0 to 1% (e.g.,0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 0.1ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% EtOH 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.1% 5 ppm to 0.05% EA 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppmto 0.1% 5 ppm to 0.05% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5ppm to 0.05% LiI 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Rh 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Fe 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Ni 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Cr 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Mo 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Zn 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Cu 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm

For example, the condensate 28 from the second condenser may have thefollowing composition.

TABLE 12 Average molecular weight 111.15 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, 10%), e.g., 0.2 ppb to 3.6% (e.g., ppt to 1000 ppm),e.g., 100 ppt to 100 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 2% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppmto 0.01% CO 0 to 2% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1%CO₂ 0 to 2% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% CH₄ 0 to1% (e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% N₂ 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% AD 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% MeOH 0 to 1% (e.g., 0.1ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% MeI 1 to 95% (e.g., 5 to90%) 10 to 85% (e.g., 50 to 85%) 70 to 83% MA 0.1 to 40% 0.5 to 20% 1 to10% H₂O 0.1 to 40% (e.g., 0.3 to 20%) 0.5 to 20% (e.g., 1 to 7%) 0.7 to5% AcOH 1 to 95% (e.g., 10 to 90%) 5 to 30% 7 to 15% FrOH 0 to 1% (e.g.,0.01 ppm to 0.1%) 0.1 ppm to 0.01% 1 ppm to 0.001% PrOH 0 to 1% (e.g.,0.01 ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% DME 0 to 1% (e.g.,0.01 ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% (CH₃)₂C═O 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 5 ppm to 0.05% EtOH 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 5 ppm to 0.05% EA 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 ppm to 0.05% 5 ppm to 0.05% EtI 0 to 1% (e.g., 0.1ppm to 0.1%) 1 ppm to 0.05% 5 ppm to 0.05% LiI 0 to 0.1% (e.g., 0.1 pptto 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 0.1 pptto 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm

For example, the noncondensable gas 30 from the second condenser mayhave the following composition.

TABLE 13 Average molecular weight 41.38 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 20ppb to 0.3% 50 ppb to 0.1% (e.g., ppt to 5%), e.g., less than 3.6% 100ppb to 200 ppm) (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppbto 0.5%) H₂ 0.01 to 5% 0.05 to 2% 0.1 to 1% CO 1 to 99% 5 to 80% 10 to70% CO₂ 0.1 to 20% 0.5 to 15% 1 to 10% CH₄ 0.1 to 20% 0.5 to 15% 1 to10% N₂ 0.1 to 20% 0.5 to 15% 1 to 10% AD 0.001 to 3% 0.01 to 1% 0.1 to0.5% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to 0.1% MeI 1to 95% 10 to 90% 20 to 80% MA 0.01 to 20% 0.1 to 10% 0.5 to 5% H₂O 0.01to 10% 0.02 to 1% 0.05 to 0.5% AcOH 0.001 to 10% 0.01 to 1% 0.05 to 0.5%FrOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% PrOH 0to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% DME 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% EtOH 0 to 1% (e.g.,0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% EA 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.1% 5 ppm to 0.05% EtI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppmto 0.1% 5 ppm to 0.05% LiI 0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 ppt to1 ppm 10 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 ppt to1 ppm 10 ppt to 0.1 ppm

As the flasher (2), a single flasher or a plurality of flashers may beused. Moreover, a portion of the volatile phase (2A) may be condensed ina condenser to form a condensate which is then recycled to the reactionstep (reactor) (1). The whole quantity of the volatile phase (2A) may befed to the distillation column of the first distillation step (3)without recycling of a portion of the volatile phase (2A) to the reactor(1).

If necessary, the catalyst component (metal catalyst component) may beseparated from the less-volatile phase (2B) by a single step or aplurality of steps and may be returned to the reaction step (1) forrecycle or reuse.

(3) First Distillation Step (Splitter Column or Distillation Column)

In the first distillation step (splitter column) (3), the volatile phase(2A) (the line 23) is separated into a first overhead (3A), a crudeacetic acid stream or side-cut crude acetic acid stream (3B), and abottom stream (3C); the first overhead (3A) (overhead gas, lower boilingpoint fraction) is withdrawn from a top or upper part of the column viaa withdrawing line 31, the crude acetic acid stream (3B) is side-cut viaa line 42 and mainly contains acetic acid, and the bottom stream (higherboiling point fraction) (3C) is withdrawn from a bottom or lower part ofthe column via a bottom line 45.

Incidentally, to the distillation column (splitter column) (3) arerecycled: a portion 66 of a third overhead (6A) (a line 61) from a thirddistillation column (6), a portion 172 of an overhead stream 171 fromthe off-gas treatment section (15) (the high-pressure absorption column(16)), and a bottom acetic acid stream 184 from the off-gas treatmentsection (15) (the low-pressure absorption column (17)).

The first overhead (3A) contains methyl iodide, water, and methylacetate and also contains acetaldehyde and carbon monoxide. The firstoverhead (3A) is fed to the separation section (9) for separatingimpurities such as acetaldehyde, and the off-gas treatment section (15).

The crude acetic acid stream (3B) (the line 42) mainly or primarilycontains acetic acid, and also contains methyl iodide, methyl acetate,water, and others.

A portion 43 of the crude acetic acid stream 42 may be returned to thefirst distillation column (splitter column) (3), the residual portion 44of the crude acetic acid stream 42 is purified by a purification section(4) for removing, for example, water and a higher boiling pointfraction, to product acetic acid with a high purity.

The bottom liquid stream (3C) (the line 45) usually contains at leastwater and acetic acid and also often contains propionic acid or others.A portion of the bottom liquid stream (3C) is returned to a bottom ofthe splitter column (3). The bottom liquid stream 45, which may containan entrained metal catalyst component (lithium iodide), is recycled tothe flasher or evaporator (2).

The first overhead (3A) contains at least one permanganate reducingcompound (PRC) and methyl iodide. The PRC contains at least by-productacetaldehyde. The first overhead (3A) usually contains methyl acetateand may practically contain acetic acid, methanol, water, dimethylether, by-products derived from acetaldehyde (e.g., an aldehyde such ascrotonaldehyde or butyraldehyde; a C₂₋₁₂alkyl iodide; an acetaldehydederivative such as a C₃₋₁₂alkanecarboxylic acid; and a C₂₋₁₂alkane).

For example, the first overhead (3A) (the line 31) may have thefollowing composition.

TABLE 14 Average molecular weight 52.19 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 20ppb to 0.3% 50 ppb to 0.1% (e.g., ppt to 5%), e.g., less than 3.6% 100ppb to 200 ppm) (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppbto 0.5%) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.03%CO 500 ppm to 10% 0.1 to 5% 0.2 to 3% CO₂ 100 ppm to 2% 500 ppm to 1%0.1% to 0.5% CH₄ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to0.3% N₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.3% AD0.01 to 1% 0.05 to 0.5% 0.1 to 0.3% MeOH 0 to 4%, e.g., 0 to 2% 100 ppmto 1% 200 ppm to 0.7% (e.g., 10 ppm to 2%) (e.g., 0.1 to 0.5%) MeI 20 to95% 30 to 90% 50 to 80% MA 1 to 40% 3 to 30% 7 to 20% H₂O 1 to 60% 5 to50% 10 to 30% AcOH 0.1 to 20% 1 to 15% 2 to 10% HI 0 to 1% (e.g., 0.1ppm to 1%) 1 ppm to 0.5% 5 ppm to 0.3% (e.g., 10 ppm to 0.1%) FrOH 0 to1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1% PrOH 0 to 0.5%(e.g., 0.1 ppm to 0.1%) 1 to 700 ppm 3 to 500 ppm (e.g., 1 to 100 ppm)(e.g., 3 to 50 ppm) DME 0 to 0.5% (e.g., 0.1 ppm to 0.1%) 0.5 to 700 ppm5 to 500 ppm (e.g., 1 to 300 ppm) (e.g., 10 to 100 ppm) (CH₃)₂C═O 0 to0.5% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EtOH 0 to 0.5%(e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EA 0 to 0.5% (e.g.,0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EtI 0 to 0.5% (e.g., 0.1 ppmto 0.1%) 1 to 100 ppm 10 to 70 ppm LiI 0 to 0.1% (e.g., 0.1 ppt to 10ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 0.1 ppt to 10ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm

For example, the crude acetic acid stream (3B) (the line 42) may havethe following composition.

The composition of the crude acetic acid stream 44 to be fed to thepurification section (purification step group or unit group) may besubstantially the same as (or similar to) the composition of the crudeacetic acid stream (3B) (the line 42).

TABLE 15 Average molecular weight 58.72 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb to ppt to1000 ppm), e.g., 100 ppt to 100 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to0.03% CO 0 to 1% (e.g., 10 ppm to 1%) 50 ppm to 0.5% 100 ppm to 0.3% CO₂0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 2 ppm to 0.1% CH₄ 0 to 1%(e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.3% N₂ 0 to 1% (e.g., 1ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.3% AD 0 to 1% (e.g., 5 ppm to 1%)10 ppm to 0.5% 20 ppm to 0.3% MeOH 0 to 2% (e.g., 10 ppm to 1.5%) 20 ppmto 1.2% 30 ppm to 0.1% (e.g., 100 ppm to 1%) (or 0.1% to 0.5%) MeI 0.1to 15% 0.5 to 10% 1 to 5% MA 0.1 to 15% 0.5 to 10% 1 to 5% H₂O 0.1 to10% 0.5 to 8% 1 to 5% AcOH 10 to 99% (e.g., 30 to 98%) 50 to 97% 60 to95% HI 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1% FrOH 0to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1% PrOH 0 to 0.5%(e.g., 0.1 ppm to 0.1%) 1 to 700 ppm 3 to 500 ppm (e.g., 1 to 100 ppm)(e.g., 3 to 50 ppm) DME 0 to 0.5% (e.g., 0.1 ppm to 0.1%) 0.5 to 700 ppm5 to 500 ppm (e.g., 1 to 300 ppm) (e.g., 10 to 100 ppm) (CH₃)₂C═O 0 to0.5% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EtOH 0 to 0.5%(e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EA 0 to 0.5% (e.g.,0.1 ppm to 0.1%) 1 to 100 ppm 10 to 70 ppm EtI 0 to 0.5% (e.g., 0.1 ppmto 0.1%) 1 to 100 ppm 10 to 70 ppm Li 0 to 0.1% (e.g., 100 ppt to 10ppm) 0.5 ppb to 50 ppm 5 ppb to 10 ppm (e.g., 1 ppb to 1 ppm) (e.g., 10ppb to 0.5 ppm) Rh 0 to 0.1% (e.g., 10 ppt to 10 ppm) 0.1 ppb to 1 ppm 1to 100 ppb Fe 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Ni 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Cr 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Mo 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Zn 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm Cu 0 to 0.1% (e.g., 0.1 ppt to 100 ppm) 1 ppt to 10 ppm 10ppt to 1 ppm

The ratio of the flow rate of the crude acetic acid stream (3B) to befed to the purification section (4) relative to that to be recycled tothe splitter column (3) [the former/the latter] may be about 100/1 to2/1 (e.g., about 25/1 to 5/1) and preferably about 15/1 to 7/1 (e.g.,about 10/1 to 8/1).

For example, the bottom liquid stream (3C) (the line 45) may have thefollowing composition.

TABLE 16 Average molecular weight 58.88 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb ppt to1000 ppm), e.g., 100 ppt to 100 to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%10 ppm to 0.01% CO 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to0.1% CO₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% CH₄0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% N₂ 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% AD 0 to 0.1% (e.g.,0.01 to 500 ppm) 0.1 to 200 ppm 1 to 100 ppm MeOH 0 to 1% (e.g., 0.1 ppmto 1%) 1 ppm to 0.2% 10 ppm to 0.1% MeI 5 ppm to 5% 50 ppm to 3% 200 ppmto 2% (e.g., 10 ppm to 2%) (e.g., 100 ppm to 1%) (e.g., 300 ppm to 0.5%)MA 0.01 to 6% 0.1 to 4% 0.5 to 3% H₂O 0.01 to 10% 0.1 to 5% 0.5 to 4%(e.g., 1 to 3%) AcOH 60 to 99.5% (e.g., 80 to 99%) 85 to 99% 90 to 98%HI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.5% 1 ppm to 0.1% FrOH 0to 1% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 5 to 100 ppm PrOH 0.1 ppm to0.1% 1 to 500 ppm 3 to 300 ppm (e.g., 5 to 100 ppm) DME 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 to 500 ppm 3 to 300 ppm (e.g., 5 to 100 ppm)(CH₃)₂C═O 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 5 to 100 ppm EtOH0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 5 to 100 ppm EA 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 5 to 100 ppm EtI 0 to 1% (e.g., 0.1ppm to 0.1%) 1 to 500 ppm 5 to 100 ppm LiI 1 ppm to 2% 3 ppm to 1.5% 5ppm to 1% (e.g., 1 ppm to 0.5%) (e.g., 5 ppm to 0.1%) (e.g., 10 to 500ppm) Rh 1 ppb to 300 ppm 10 ppb to 100 ppm 100 ppb to 50 ppm Fe 0 to 1%(e.g., 0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% Ni 0 to 0.5% (e.g.,0.1 ppm to 0.2%) 1 ppm to 0.1% 10 to 400 ppm Cr 0 to 0.5% (e.g., 0.1 ppmto 0.2%) 1 ppm to 0.1% 10 to 400 ppm Mo 0 to 0.5% (e.g., 0.01 ppm to0.1%) 0.1 to 400 ppm 1 to 200 ppm Zn 0 to 1% (e.g., 0.1 ppm to 4000 ppm)1 ppm to 0.2% 10 ppm to 0.1% Cu 0 to 0.1% (e.g., 0.001 to 200 ppm) 0.01to 100 ppm 0.1 to 20 ppm

As the distillation column (splitter column), there may be used a platecolumn, a packed column, or other columns. The liquid stream (3C) may bedischarged. A portion or whole of the liquid stream (3C) may be returnedto the splitter column (3) or may be recycled to the reaction step (thereactor) (1).

(4) Purification Section (Purification Step Group or Unit Group)

The crude acetic acid stream (3B) (a line 44) contains impurities suchas a lower boiling point impurity, a higher boiling point impurity, andan ionic iodine compound. In order to separate and remove theseimpurities for purification, the crude acetic acid stream (3B) is fed tothe purification section (purification step group or unit group) (4).For example, the purification section (4) may comprise the followingsteps (5), (6), (7), and (8): (5) a dehydration step (a dehydrationdistillation column) for mainly removing water from the crude aceticacid stream; (6) a higher boiling component (or fraction) removing step(a heavy end column or a higher boiling component (or fraction)distillation column) for removing a higher boiling component (orfraction) from the crude acetic acid stream; (7) a purification step (apurification distillation column) for further removing impurities fromthe crude acetic acid stream; and (8) an ion exchange step forseparating an iodine compound from the crude acetic acid stream. Thearrangement of the dehydration step (5), the higher boiling componentremoving step (6), the purification step (7), and the ion exchange step(8) is not limited to this order, and, for example, after the ionexchange step (8), the dehydration step (5), the higher boiling point(or heavy) component removing step (6), and the purification step (7)may be arranged in this order. After the dehydration step (5) and thehigher boiling component removing step (6), the ion exchange step (8)and then the purification step (7) may be arranged in this order. Afterthe dehydration step (5), the higher boiling component removing step(6), and the purification step (7), the ion exchange step (8) may becarried out. The purification section (4) usually comprises at least thedehydration step (5) among the steps (5) to (8). The purification step(7) is not necessarily needed.

(5) Dehydration Step (Dehydration Distillation Column)

In the dehydration step (5), the crude acetic acid stream (3B) (the line44) is distilled in the second distillation column (dehydrationdistillation column) to form a second overhead (5A) rich in water and abottom acetic acid stream (5B) rich in acetic acid; the second overhead(5A) is withdrawn from a top or upper part of the column via awithdrawing line 51, and the bottom acetic acid stream (5B) is withdrawnfrom a bottom or lower part of the column via a bottom line 56. Aportion of the bottom acetic acid stream (5B) is heated by a heatingunit and is then returned to the dehydration step (dehydrationdistillation column) (5), and the residual portion of the bottom aceticacid stream (5B) is fed to the third distillation column (heavy endcolumn or higher boiling component distillation column) (6).

The second overhead (5A) is cooled in a condenser and is then introducedto a hold tank T2 to form a condensate 52 and a gaseous phase 55. Aportion 53 of the condensate 52 is returned to the second distillationcolumn (5) for reflux, and another portion of the condensate is mixedwith the less-volatile phase (2B) via a line 54, and the mixture isrecycled to the reactor (1). The gaseous phase (noncondensable gas(off-gas)) 55 from the hold tank T2, which is rich in carbon monoxide,is fed to the off-gas treatment section (15).

For example, the second overhead (5A) (the line 51) may have thefollowing composition.

The composition of the condensates 52, 53 of the second overhead (5A)may be substantially the same as (or similar to) the composition of thesecond overhead (5A). The compositions of the condensates 52, 53 mayhave a component ratio (content of each component) obtained orcalculated by subtracting the component ratio of the gaseous phase(noncondensable gas) 55 from the hold tank T2 from the component ratioof the second overhead (5A).

TABLE 17 Average molecular weight 56.08 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 ppt20 ppb to 0.3% 50 ppb to 0.1% (e.g., to 5%), e.g., less than 3.6% (e.g.,100 ppb to 200 ppm) 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 ppb to 500 ppm 1 ppb to100 ppm (e.g., 0.1 to 500 ppm) (e.g., 1 to 100 ppm) CO 0 to 1% (e.g.,0.1 ppm to 1%) 0.1 ppb to 0.5% 10 ppb to 0.1% (e.g., 1 ppm to 0.5%)(e.g., 10 ppm to 0.1%) CO₂ 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 ppb to500 ppm 1 ppb to 100 ppm (e.g., 0.1 to 500 ppm) (e.g., 1 to 100 ppm) CH₄0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 ppb to 500 ppm 1 ppb to 100 ppm(e.g., 0.1 to 500 ppm) (e.g., 1 to 100 ppm) N₂ 0 to 5%, 0 to 2%, 0 to 1%0.1 to 500 ppm 1 to 100 ppm (e.g., 0.01 ppm to 0.1%) AD 0.1 ppm to 1%,e.g., 1 ppm to 0.2%, 1 ppm to 0.1%, e.g., 1 ppm to 0.3% e.g., 10 ppm to0.1% 50 to 500 ppm MeOH 0 to 2% (e.g., 10 ppm to 2%) 100 ppm to 1% 200ppm to 0.5%, e.g., 0.1 to 0.5% MeI 0.1 to 30% 1 to 20% 3 to 15% MA 0.1to 20% 1 to 15% 2 to 12% H₂O 0.1 to 20% 1 to 15% 2 to 10% AcOH 10 to 95%(e.g., 30 to 90%) 50 to 85% 60 to 85% HI 0 to 1% (e.g., 0.1 ppm to 1%),10 ppb to 0.5% 0.1 ppm to 0.1% e.g., 0.1 ppb to 1% (e.g., 1 ppm to 0.5%)(e.g., 10 ppm to 0.1%) FrOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5%10 ppm to 0.1% PrOH 0 to 0.3% (e.g., 0.1 ppm to 0.1%) 1 to 700 ppm 3 to500 ppm (e.g., 1 to 100 ppm) (e.g., 3 to 200 ppm), e.g., 3 to 50 ppm DME0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 to 500 ppm (CH₃)₂C═O 0to 0.5% (e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EtOH 0 to0.5% (e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EA 0 to 0.5%(e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EtI 0 to 0.5% (e.g.,0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm Li 0 to 0.1% (e.g., 0.1 pptto 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 0.1 pptto 10 ppm) 1 ppt to 1 ppm 10 ppt to 0.1 ppm

The composition of the condensate 52 from the hold tank T2 may besubstantially the same as (or similar to) the composition of the secondoverhead (5A) (the line 51).

For example, the gaseous phase (noncondensable gas) 55 from the holdtank T2 may have the following composition.

Incidentally, as described above, in a case where an inactive gas (suchas nitrogen gas N₂) is introduced to regulate the pressure of thedistillation column (5) and/or to prevent an organic matter fromentering the measuring instrument, the composition of the inactive gas(such as nitrogen gas N₂) shown in the following Table 18 drasticallyincreases.

TABLE 18 Average molecular weight 27.90 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 ppt20 ppb to 0.3% 50 ppb to 0.1% (e.g., to 5%), e.g., less than 3.6% (e.g.,100 ppb to 200 ppm) 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm 1 to 300 ppm CO1 to 99.9% 50 to 99.8% 70 to 99.6% CO₂ 0 to 1% (e.g., 0.01 ppm to 0.5%)0.1 ppm to 0.1% 1 to 500 ppm CH₄ 1 ppm to 10% 10 ppm to 3% 100 ppm to 1%N₂ 1 ppm to 10% 10 ppm to 3% 100 ppm to 1% AD 0 to 1% (e.g., 0.1 ppm to0.5%) 1 ppm to 0.1% 10 to 500 ppm MeOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1ppm to 0.1% 10 to 500 ppm MeI 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5%10 pm to 0.1% MA 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 pm to 0.1%H₂O 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 pm to 0.1% AcOH 0 to 1%(e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 pm to 0.1% HI 0 to 0.5% (e.g., 0.01ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% FrOH 0 to 0.5% (e.g., 0.01ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% PrOH 0 to 0.5% (e.g., 0.01ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% DME 0 to 0.5% (e.g., 0.01ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to 0.01% (CH₃)₂C═O 0 to 0.5% (e.g.,0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EtOH 0 to 0.5% (e.g., 0.01ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppm EA 0 to 0.5% (e.g., 0.01 ppm to0.1%) 0.1 to 100 ppm 1 to 50 ppm EtI 0 to 0.5% (e.g., 0.01 ppm to 0.1%)0.1 to 100 ppm 1 to 50 ppm Li 0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 pptto 1 ppm 10 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 0.1 ppt to 10 ppm) 1 pptto 1 ppm 10 ppt to 0.1 ppm

For example, the bottom acetic acid stream (5B) (the line 56) may havethe following composition.

TABLE 19 Average molecular weight 59.99 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb to 1 ppt to1000 ppm), e.g., 100 ppt to 100 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to10 ppm CO 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppmCO₂ 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm CH₄ 0 to0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm N₂ 0 to 0.1%(e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm AD 0 to 0.05% (e.g.,0.001 to 50 ppm) 0.01 to 20 ppm 0.1 to 10 ppm MeOH 0 to 0.1% (e.g.,0.001 to 100 ppm) 0.01 to 10 ppm 0.1 to 5 ppm MeI 0 to 0.01% (e.g., 0.01to 10 ppb) 0.05 to 200 ppb 0.2 to 50 ppb (e.g., 0.1 to 5 ppb) (e.g., 0.2to 10 ppb), e.g., 0.3 to 2 ppb MA 0 to 0.1% (e.g., 0.001 ppm to 0.1%)0.01 to 100 ppm 0.1 to 50 ppm H₂O 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to0.5% 100 ppm to 0.1% AcOH 98 to 100% 99 to 99.999% 99.5 to 99.99% HI 0to 0.1% (e.g., 0.01 ppb to 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppmFrOH 0 to 0.1% (e.g., 0.1 to 500 ppm) 1 to 100 ppm 5 to 50 ppm PrOH 0 to0.2% (e.g., 5 ppm to 0.2%) 30 ppm to 0.1% 70 to 500 ppm (e.g., 100 to250 ppm) DME 0 to 0.1% (e.g., 1 ppb to 10 ppm) 10 ppb to 5 ppm 50 ppb to1 ppm (CH₃)₂C═O 0 to 0.1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10ppb to 1 ppm EtOH 0 to 0.1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm10 ppb to 1 ppm EA 0 to 0.1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm10 ppb to 1 ppm EtI 0 to 0.1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm10 ppb to 1 ppm Li 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 1 ppt to 10 ppm) 10 ppt to 1 ppm 100ppt to 0.1 ppm

In the second distillation step (5), in order to convert hydrogen iodidein the crude acetic acid stream 44 into methyl iodide which is distilledout as the second overhead (5A) (the line 51), methanol 3 may be addedto one or a plurality of sites of the second distillation column(dehydration distillation column) (5). Further, the bottom acetic acidstream 56 from the second distillation step (5) may be mixed with apotassium hydroxide aqueous solution 57 to allow hydrogen iodide toreact with potassium hydroxide for removing hydrogen iodide as potassiumiodide. The bottom acetic acid stream 58 which has been treated withpotassium hydroxide may be distilled in the third distillation step (6)for mainly separating and removing a higher boiling component (orfraction).

In order to remove hydrogen iodide, a methanol source, for example, atleast one component selected from the group consisting of methanol,methyl acetate, and dimethyl ether, may be added to the distillationcolumn. Potassium hydroxide is used in the above embodiment. However,other alkali metal components may also be used. The alkali metalcomponents may include, for example, an alkali metal hydroxide (e.g.,sodium hydroxide), an alkali metal carbonate, and an alkali metalacetate (such as sodium acetate or potassium acetate).

The composition of the methanol 3 is substantially the same as describedabove.

For example, the potassium hydroxide aqueous solution 57 may have thefollowing composition.

TABLE 20 Average molecular weight 25.94 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 ppt 10ppt to 300 ppm, e.g., ppb to 10%), e.g., 0.2 to 1000 ppm), e.g., less100 ppt to 100 ppm ppb to 3.6% (e.g., 1 ppb than 700 ppm (e.g., 1 ppt to2%) to 500 ppm) H₂O 40 to 99.9% 50 to 99% 55 to 90% KOH 0.1 to 60% 1 to50% 10 to 45%

For example, the bottom acetic acid stream 58 may have the followingcomposition.

TABLE 21 Average molecular weight 59.98 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% (e.g., 1 ppb to ppt to1000 ppm), e.g., 100 ppt to 100 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to10 ppm CO 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppmCO₂ 0 to 0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm CH₄ 0 to0.1% (e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm N₂ 0 to 0.1%(e.g., 0.01 to 100 ppm) 0.1 to 50 ppm 1 to 10 ppm AD 0 to 0.1% (e.g.,0.001 to 50 ppm) 0.01 to 20 ppm 0.1 to 10 ppm MeOH 0 to 0.1% (e.g.,0.001 to 100 ppm) 0.01 to 10 ppm 0.1 to 5 ppm MeI 0 to 0.01% (e.g., 0.01to 10 ppb) 0.1 to 5 ppb 0.3 to 2 ppb MA 0 to 0.1% (e.g., 0.001 ppm to0.1%) 0.01 to 100 ppm 0.1 to 50 ppm H₂O 0 to 1% (e.g., 1 ppm to 1%) 10ppm to 0.5% 100 ppm to 0.1% AcOH 98 to 99.999% 99 to 99.99% 99.5 to99.9% HI 0 to 0.1% (e.g., 0.01 ppb to 100 ppm) 0.1 ppb to 10 ppm 1 ppbto 1 ppm FrOH 0 to 0.1% (e.g., 0.1 to 500 ppm) 1 to 100 ppm 5 to 50 ppmPrOH 0 to 0.2% (e.g., 5 ppm to 0.2%) 30 ppm to 0.1% 100 to 250 ppm(e.g., 70 to 500 ppm) KOH 0 to 0.1% (e.g., 1 ppm to 0.1%) 5 to 500 ppm10 to 100 ppm DME 0 to 0.1% (e.g., 1 ppb to 10 ppm) 10 ppb to 5 ppm 50ppb to 1 ppm (CH₃)₂C═O 0 to 0.1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1ppm 1 ppb to 0.1 ppm EtOH 0 to 0.1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppbto 1 ppm 1 ppb to 0.1 ppm EA 0 to 0.1% (e.g., 0.01 ppb to 10 ppm) 0.1ppb to 1 ppm 1 ppb to 0.1 ppm EtI 0 to 0.1% (e.g., 0.01 ppb to 10 ppm)0.1 ppb to 1 ppm 1 ppb to 0.1 ppm Li 0 to 0.1% (e.g., 1 ppt to 10 ppm)10 ppt to 1 ppm 100 ppt to 0.1 ppm Rh 0 to 0.1% (e.g., 1 ppt to 10 ppm)10 ppt to 1 ppm 100 ppt to 0.1 ppm

The dehydration step (dehydration distillation column) (5) may comprisea single step (a distillation column) or may comprise a plurality ofsteps (distillation columns) for distilling the bottom stream (5B) byone or a plurality of succeeding step(s) (distillation column(s)). Forexample, a portion of the bottom stream (5B) may be returned to thedehydration step (dehydration distillation column) (5), while theresidual portion of the bottom stream (5B) may be fed to the succeedingdehydration step (dehydration distillation column) (5). As thedistillation column(s) of the second distillation step (5), there may beused a plate column, a packed column, or other columns.

(6) Higher Boiling Component Removing Step (Heavy End Column)

The bottom acetic acid stream (5B) still contains a higher boilingcomponent such as propionic acid, although the bottom acetic acidstream, from which a lower boiling component (or fraction) has beenremoved, has a significantly improved acetic acid purity. Thus, thebottom acetic acid stream (a line 56 or a line 58) is subjected to thethird distillation step (heavy end column) (6) for removing the higherboiling component. Specifically, in the third distillation step (heavyend column) (6), the bottom acetic acid stream (5B) is distilled to forma third overhead (6A) (a line 61) rich in acetic acid, an acetic acidstream (6B) (a line 67) rich in acetic acid, and a bottom stream (6C) (aline 68) rich in a higher boiling component (or fraction) containingacetic acid; the third overhead (6A) is withdrawn from a top or upperpart of the column, the acetic acid stream (6B) is side-cut at a portionupper than a middle portion of the column, and the bottom stream (6C) iswithdrawn from a bottom or lower part of the column.

The side-cut acetic acid stream (6B) (the line 67) is further purifiedin a fourth distillation step (purification column) (7) for removal ofimpurities. A side-cut acetic acid stream (7B) from the fourthdistillation step (purification column) (7) id fed to the ion exchangestep (8). A portion of the side-cut acetic acid stream (7B) (a line 75)is mixed with the bottom stream (5B) of the second distillation step(dehydration distillation column) (5) via a line 76.

The third overhead (6A) is cooled and condensed in a condenser to formacondensate 62 which is then held in a hold tank T3. A first portion ofthe condensate 62 is returned for reflux to an upper part of the higherboiling point removing step (heavy end column) (6) via a reflux line 63.A second portion of the condensate 62 is recycled to the distillationcolumn (dehydration distillation column) (5) via a line 64. A thirdportion of the condensate 62 is fed to a diffusion step (diffusioncolumn) (18) of the off-gas treatment section (15) via a line 65 and theflash evaporator (2) via a line 66. A noncondensable gas from the holdtank T3 may be fed to the reactor (1) or the evaporator (2).

A portion of the bottom stream (6C) (the line 68) containing acetic acidis returned to the third distillation step (heavy end column) (6), andthe residual portion of the bottom stream (6C) (the line 68) is fed toan incineration unit (not shown) via a line 69.

For example, the third overhead (6A) may have the following composition.

The compositions of the reflux liquid (lines 62, 63) and the condensate(line 64, line 65) may have a composition ratio (content of eachcomponent) obtained or calculated by subtracting a component ratio of anoncondensable gas in the condenser and a noncondensable gas in the holdtank T3 from the component ratio of the third overhead (6A).

TABLE 22 Average molecular weight 59.58 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb 100 ppb to0.1%), or (e.g., 1 to 100 ppm), or to 2%), or less than 7% (e.g., 1 20ppb to 0.3% 50 ppb to 0.1% (e.g., ppt to 5%), e.g., less than 3.6% 100ppb to 200 ppm) (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppbto 0.5%) H₂ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10ppm CO 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 10 ppm to 0.01%CO₂ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm CH₄0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm N₂ 0 to1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm AD 0 to 1%(e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 0.2 to 50 ppm, 0.5 to 50 ppmMeOH 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 0.5 to 50 ppm MeI 0to 1% (e.g., 0.01 ppm to 0.1%) 0.001 to 300 ppm 0.003 to 50 ppm (e.g.,0.1 to 100 ppm) (e.g., 0.5 to 10 ppm) MA 0 to 1% (e.g., 1 ppm to 1%) 0.1ppm to 0.5% 1 to 750 ppm (e.g., 10 ppm to 0.1%) (e.g., 50 to 500 ppm)H₂O 10 ppm to 2% 100 ppm to 1% 0.1% to 0.5% AcOH 90 to 99.99% 98 to99.9% 99 to 99.8% HI 0 to 1% (e.g., 0.1 ppb to 0.1%) 1 ppb to 100 ppm 2ppb to 10 ppm (e.g., 10 ppb to 10 ppm) FrOH 0 to 1% (e.g., 0.1 ppm to0.5%) 1 ppm to 0.1% 10 to 500 ppm PrOH 0 to 1% (e.g., 0.01 ppm to 0.1%)0.1 to 200 ppm 1 to 50 ppm DME 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to200 ppm 1 to 50 ppm (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppbto 1 ppm 1 ppb to 0.1 ppm EtOH 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1ppb to 1 ppm 1 ppb to 0.1 ppm EA 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1ppb to 1 ppm 1 ppb to 0.1 ppm EtI 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1ppb to 1 ppm 1 ppb to 0.1 ppm Li 0 to 0.1% (e.g., 0.01 ppt to 1 ppm) 0.1ppt to 0.1 ppm 1 ppt to 0.01 ppm Rh 0 to 0.1% (e.g., 0.01 ppt to 1 ppm)0.1 ppt to 0.1 ppm 1 ppt to 0.01 ppm

For example, the side-cut acetic acid stream (6B) (the line 67) may havethe following composition.

TABLE 23 Average molecular weight 60.01 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, 10%), e.g., 0.2 ppb to 3.6% ppt to 1000 ppm), e.g.,100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1to 10 ppm CO 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 10 ppm to0.01% CO₂ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10ppm CH₄ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppmN₂ 0 to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm AD 0to 1% (e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm MeOH 0 to1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm MeI 0 to1% (e.g., 0.01 ppb to 20 ppb) 0.1 ppb to 10 ppm 0.5 to 5 ppb MA 0 to 1%(e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm H₂O 0 to 1%(e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% AcOH 99 to 100% 99.8to 99.999% 99.9 to 99.99% HI 0 to 1% (e.g., 0.01 to 100 ppb) 0.1 to 10ppb 0.5 to 5 ppb FrOH 0 to 1% (e.g., 0.1 to 500 ppm) 1 to 100 ppm 5 to50 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 500 ppm 10 to 250 ppmDME 50 ppm or less (1 ppt to 50 ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm(CH₃)₂C═O 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to0.1 ppm EtOH 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppbto 0.1 ppm EA 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppbto 0.1 ppm EtI 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppbto 0.1 ppm Li 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100 pptto 0.01 ppm Rh 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 50 pptto 0.01 ppm

For example, the bottom stream (6C) (the line 68) may have the followingcomposition.

TABLE 24 Average molecular weight 59.70 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% 1 ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to10 ppm 1 ppb to 1 ppm CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm1 ppb to 10 ppm CO₂ 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 1ppb to 1 ppm CH₄ 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 1 ppbto 1 ppm N₂ 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 1 ppb to 1ppm AD 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 1 ppb to 1 ppmMeOH 0 to 1% (e.g., 1 ppt to 100 ppm) 10 ppt to 10 ppm 1 ppb to 1 ppmMeI 0 to 1% (e.g., 1 ppt to 10 ppm) 10 ppt to 10 ppm 0.1 ppb to 1 ppm MA0 to 1% (e.g., 1 ppt to 100 ppm) 0.01 to 10 ppm 0.1 ppm to 1 ppm H₂O 0to 1% (e.g., 1 ppm to 0.5%) 5 ppm to 0.1% 20 ppm to 0.02% AcOH 80 to 99%85 to 98% 90 to 95% HI 0 to 1% (e.g., 0.01 ppb to 100 ppm) 0.1 ppb to 10ppm 0.5 ppb to 1 ppm FrOH 0 to 1% (e.g., 0.1 ppb to 0.5%) 1 ppb to 0.1%10 ppb to 500 ppm PrOH 0 to 1% (e.g., 10 ppm to 10%) 50 ppm to 1% 100ppm to 0.1% DME 1 ppm or less (1 ppt to 1 ppm) 1 ppb to 10 ppm 10 ppb to1 ppm (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1ppb to 0.1 ppm KOH 0.01 to 40% 0.1 to 20% 1 to 15%, e.g., 3 to 10% EtOH0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EA0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EtI0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm Li0 to 0.1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm Rh0 to 0.1% (e.g., 0.1 ppb to 50 ppm) 1 ppb to 2 ppm 10 ppb to 1 ppm Fe 0to 0.1% (e.g., 100 ppt to 100 ppm) 1000 ppt to 50 ppm 1000 ppt to 10 ppmNi 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 1 ppm 1000 ppt to 0.5ppm Cr 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 1 ppm 1000 ppt to0.5 ppm Mo 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 1 ppm 1000 pptto 0.5 ppm Zn 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 1 ppm 1000ppt to 0.5 ppm Cu 0 to 0.1% (e.g., 10 ppt to 10 ppm) 100 ppt to 1 ppm1000 ppt to 0.5 ppm

The higher boiling component removing step (heavy end column) (6) mayalso comprise a single step (a distillation column) or a plurality ofsteps (distillation columns). For example, a portion of the bottomstream (6C) may be returned to the higher boiling component removingstep (heavy end column) (6), while the residual portion of the bottomstream (6C) may be fed to the succeeding higher boiling componentremoving step (heavy end column) (6). The bottom stream(s) (6C) from oneor a plurality of higher boiling component removing steps (heavy endcolumns) (in particular, the last higher boiling component removingstep) may be discharged as a waste fluid. As the distillation column(s)of the third distillation step (6), there may be used a plate column, apacked column, or other columns.

(7) Purification step

In the purification step (purification distillation column) (7), theacetic acid stream (6B) (the line 67) from the higher boiling componentremoving step (third distillation column) (6) is distilled to form afourth overhead (7A) rich in a lower boiling component (or fraction), apurified acetic acid (7B), and a bottom stream (7C) containing a higherboiling component (or fraction); the fourth overhead (7A) is withdrawnfrom a top or upper part of the column via a withdrawing line 71, thepurified acetic acid (7B) is side-cut via a withdrawing line 75, and thebottom stream (7C) is withdrawn from a bottom or lower part of thecolumn via a bottom line 77.

The fourth overhead (7A) is cooled and condensed in a condenser on theline 71 to form a condensate and a noncondensable gas. A portion of thecondensate from the condenser is returned to the purification step(purification distillation column) (7) via a line 72 for reflux, and theresidual portion of the condensate is fed to an incineration unit (notshown) via a line 73. The noncondensable gas (off-gas) is fed to anincineration unit (not shown) via a line 74. The noncondensable gas(off-gas) may be recycled to the reaction system.

A first portion of the bottom stream (7C) is steam-heated by a reboiler(heat exchanger) on a line 80 by a portion of the third overhead (6A)(the line 61) from the higher boiling component removing step (thirddistillation column) (6) and is recycled to the purification step(purification distillation column) (7). Specifically, the heat energy ofthe portion of the third overhead (6A) is supplied to the first portionof the bottom stream (7C) as a heat source in the purification step(purification distillation column) (7).

A second portion of the bottom stream (7C) is heated by a reboiler(heater) on a line 78, and the resulting vapor is recycled to thepurification step (purification distillation column) (7).

The portion of the third overhead (6A) cooled by the reboiler (heatexchanger) on the line 80 is held in a hold tank T4 and is then mixedwith the residual portion of the bottom stream (7C), and the mixture isalso recycled to the higher boiling component removing step (higherboiling component distillation column) (6) via a line 79 to remove ahigher boiling component.

The side-cut purified acetic acid (7B) (the line 75) is cooled in acondenser or cooler and is then fed to the ion exchange step (8) via aline 81, and the treated purified acetic acid may be held in a producttank T5.

For example, the fourth overhead (7A) (the line 71) may have thefollowing composition.

TABLE 25 Range Preferred range More preferred range O₂ 10% or less(e.g., 10 ppb to 10%), 30 ppb to 1% (e.g., 500 ppb to 500 ppm e.g., 10ppb to 3.6% (e.g., 20 ppb 100 ppb to 0.1%), or (e.g., 1 to 100 ppm), orto 2%), or less than 7% (e.g., 1 ppt to 20 ppb to 0.3% 50 ppb to 0.1%(e.g., 5%), e.g., less than 3.6% (e.g., 0.1 100 ppb to 200 ppm) ppb to2%), e.g., 1 ppb to 1% (e.g., 10 ppb to 0.5%) H₂ 0 to 1% (e.g., 0.001ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm CO 0 to 1% (e.g., 0.01 ppm to1%) 0.1 ppm to 0.1% 10 ppm to 0.01% CO₂ 0 to 1% (e.g., 0.001 ppm to0.1%) 0.01 to 100 ppm 0.1 to 10 ppm CH₄ 0 to 1% (e.g., 0.001 ppm to0.1%) 0.01 to 100 ppm 0.1 to 10 ppm N₂ 0 to 1% (e.g., 0.001 ppm to 0.1%)0.01 to 100 ppm 0.1 to 10 ppm AD 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to100 ppm 0.5 to 50 ppm MeOH 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 100ppm 0.5 to 50 ppm MeI 0 to 1% (e.g., 1 ppb to 0.1%) 0.01 to 100 ppm 0.1to 10 ppm MA 0 to 5% (e.g., 1 ppm to 3%) 10 ppm to 2% 100 ppm to 1% H₂O0.1 to 50% 1 to 30% 10 to 30% AcOH 50 to 99% 60 to 95% 70 to 90% HI 0 to1% (e.g., 0.01 ppb to 0.1%) 0.1 ppb to 100 ppm 1 ppb to 10 ppm FrOH 0 to1% (e.g., 10 ppm to 3%) 100 ppm to 2% 0.1% to 1% PrOH 0 to 1% (e.g.,0.01 ppm to 0.1%) 0.1 to 200 ppm 1 to 50 ppm DME 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 200 ppm 100 ppb to 50 ppm (CH₃)₂C═O 0 to 1% (e.g., 0.01ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EtOH 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EA 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EtI 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm Li 0 to 0.1%(e.g., 0.01 ppt to 0.1 ppm) 0.1 ppt to 0.01 ppm 1 ppt to 0.001 ppm Rh 0to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1 ppt to 0.01 ppm 1 ppt to 0.001ppm

For example, the composition of the condensates 72, 73 may besubstantially the same as (or similar to) the composition of the fourthoverhead (7A) (the line 71). The compositions of the condensates 72, 73may have a component ratio (content of each component) obtained orcalculated by subtracting the component ratio of an off-gas 74 from thecomponent ratio of the fourth overhead (7A) (the line 71).

For example, the off-gas 74 may have the following composition.

Incidentally, as described above, in a case where the inactive gas (suchas nitrogen gas or carbon monoxide gas) purge is performed to regulatethe pressure of the distillation column (7) and/or to protect themeasuring instrument, the nitrogen or other concentrations as shown inthe following Table 26 drastically increases according to the amount ofthe inactive gas introduced.

TABLE 26 Range Preferred range More preferred range O₂ 10% or less(e.g., 10 ppb to 10%), 30 ppb to 1% (e.g., 500 ppb to 500 ppm e.g., 10ppb to 3.6% (e.g., 20 ppb to 100 ppb to 0.1%), or (e.g., 1 to 100 ppm),or 2%), or less than 7% (e.g., 1 ppt to 20 ppb to 0.3% 50 ppb to 0.1%(e.g., 5%), e.g., less than 3.6% (e.g., 0.1 100 ppb to 200 ppm) ppb to2%), e.g., 1 ppb to 1% (e.g., 10 ppb to 0.5%) H₂ 0 to 1% (e.g., 1 ppb to1%) 0.01 ppm to 0.5% 0.1 to 500 ppm CO 0 to 1% (e.g., 1 ppm to 99.9%) 10ppm to 99% 100 ppm to 98% CO₂ 0 to 1% (e.g., 1 ppb to 1%) 0.01 ppm to0.5% 0.1 to 500 ppm CH₄ 0 to 1% (e.g., 1 ppb to 1%) 0.01 ppm to 0.5% 0.1to 500 ppm N₂ 0 to 1% (e.g., 1 ppm to 80%) 10 ppm to 75% 100 ppm to 70%AD 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 to 500 ppm MeOH 0 to1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 to 500 ppm MeI 1 ppm to 5%10 ppm to 3% 100 ppm to 1% MA 1 ppm to 20% 10 ppm to 5% 100 ppm to 1%H₂O 10 ppm to 30% 100 ppm to 20% 0.1 to 10% AcOH 10 ppm to 30% 100 ppmto 20% 0.1 to 10% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100ppb to 0.001% FrOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to0.1% PrOH 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to0.01% DME 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 ppm to 0.05% 1 ppm to0.01% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1ppb to 0.1 ppm EtOH 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm1 ppb to 0.1 ppm EA 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm1 ppb to 0.1 ppm EtI 0 to 1% (e.g., 0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm1 ppb to 0.1 ppm Li 0 to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1 ppt to0.01 ppm 1 ppt to 0.001 ppm Rh 0 to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1ppt to 0.01 ppm 1 ppt to 0.001 ppm

For example, the side-cut purified acetic acid (7B) (the line 75) mayhave the following composition.

TABLE 27 Range Preferred range More preferred range O₂ 10% or less(e.g., 0.1 ppb to 10%), less than 1% (e.g., 1 10 ppt to 300 ppm, e.g.,e.g., 0.2 ppb to 3.6% ppt to 1000 ppm), e.g., 100 ppt to 100 ppm (e.g.,1 ppb to 2%) less than 700 ppm (e.g., 1 ppt to 500 ppm) H₂ 0 to 1%(e.g., 0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm CO 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 to 100 ppm CO₂ 0 to 1% (e.g.,0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm CH₄ 0 to 1% (e.g.,0.001 ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm N₂ 0 to 1% (e.g., 0.001ppm to 0.1%) 0.01 to 100 ppm 0.1 to 10 ppm AD 0 to 1% (e.g., 0.001 to100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm MeOH 0 to 1% (e.g., 0.1 ppb to 100ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm MeI 0 to 0.1% (e.g., 1 ppt to 20ppb) 10 ppt to 10 ppb 100 ppt to 5 ppb MA 0 to 1% (e.g., 1 ppb to 100ppm) 5 ppb to 50 ppm 50 ppb to 25 ppm (e.g., 10 ppb to 10 ppm) (e.g.,100 ppb to 5 ppm) H₂O 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 50ppm to 0.1% AcOH 99.8 to 100% 99.9 to 99.999% 99.95 to 99.99% HI 0 to0.1% (e.g., 1 ppt to 20 ppb) 10 ppt to 10 ppb 100 ppt to 5 ppb TOI 0 to1% (e.g., 0.1 ppb to 0.1%) 1 ppb to 100 ppm 10 ppb to 10 ppm HexI 0 to0.1% (e.g., 1 ppt to 20 ppb) 10 ppt to 10 ppb 100 ppt to 5 ppb FrOH 0 to1% (e.g., 0.1 to 500 ppm) 1 to 100 ppm 5 to 50 ppm PrOH 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 to 500 ppm 10 to 250 ppm DME 0 to 1% (e.g., 1 ppt to50 ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm AcA 0 to 1% (e.g., 0.1 ppb to0.1%) 1 ppb to 100 ppm 10 ppb to 10 ppm (CH₃)₂C═O 0 to 1% (e.g., 0.01ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EtOH 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EA 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm EtI 0 to 1% (e.g.,0.01 ppb to 10 ppm) 0.1 ppb to 1 ppm 1 ppb to 0.1 ppm Li 0 to 0.1%(e.g., 0.01 to 100 ppb) 0.1 to 10 ppb 0.5 to 5 ppb Rh 0 to 0.1% (e.g., 1ppt to 50 ppb) 10 ppt to 10 ppb 50 ppt to 3 ppb

Incidentally, cooling of the side-cut purified acetic acid (7B) in thecondenser seems to hardly generate a noncondensable gas. For example, incooling the purified acetic acid (7B), the noncondensable gas has avolume proportion of 1% or less (e.g., 0.1% or less) in the whole fluid(or phases). Thus, the purified acetic acid (8B) (the line 81) from thecondenser is different from the above side-cut purified acetic acid (8B)in only temperature from each other (for example, the former purifiedacetic acid (8B) has a temperature of 17 to 60° C.) and havesubstantially the same (or similar) composition.

The composition of the bottom stream (7C) (lines 77, 78, 79) issubstantially the same as (or similar to) the composition of side-cutpurified acetic acid (7B) (the line 75), for example, except Li and/orRh. For example, the concentrations of Li and Rh are as follows.

Incidentally, oxygen contained in the distillation column (7) causescoloring of the side-cut purified acetic acid (7B) (the line 75) andstronger coloring of the bottom stream (7C).

TABLE 28 Range Preferred range More preferred range Li 1 ppm or less(0.1 ppb to 1000 ppb) 1 to 100 ppb 5 to 50 ppb Rh 1 ppm or less (10 pptto 500 ppb) 100 ppt to 100 ppb 500 ppt to 30 ppb

(8) Ion Exchange Step

In order to separate an iodine compound from the acetic acid stream (7B)from the purification step (purification distillation column) (7), theacetic acid stream (7B) is cooled and treated in an ion exchange tank(8) to give purified acetic acid stream (8A). The purified acetic acidstream (8A) is sent to the product tank T5 for storage via a line 82.

For example, the concentration of oxygen and the composition of othercomponents of the acetic acid stream 82 treated in the ion exchange tank(8) may be substantially the same as (or similar to) those of theside-cut purified acetic acid (7B) (the line 75), except for componentsto be removed by ion exchange.

As an ion exchanger in the ion exchange tank (8), there may be used anion exchanger capable of removing or adsorbing an iodine compound (e.g.,a zeolite, an activated carbon, and an ion exchange resin),particularly, a cation exchange resin. The cation exchange resin may bea slightly acidic cation exchange resin. A preferred cation exchangeresin may include a strongly acidic cation exchange resin, for example,a macroreticular ion exchange resin. The ion exchanger may have at leasta part of an active site replaced with or exchanged for a metal whichmay include silver Ag, mercury Hg, and/or copper Cu. Examples of theactive site may include a cationic group such as a sulfone group, acarboxyl group, a phenolic hydroxyl group, and a phosphine group. Inother words, the ion exchanger may be a metal-supported ion exchanger.For example, the ion exchanger may be a metal-supported ion exchanger inwhich about 10 to 80% by mol, preferably about 25 to 75% by mol, andmore preferably 30 to 70% by mol of the active site is replaced with themetal (e.g., silver Ag).

The ion exchanger (e.g., a silver-supported ion exchange resin) isusually fed or filled in the ion exchange column or treatment unit. Thecontact of the acetic acid stream with the ion exchanger (preferably therunning or passing of the acetic acid stream through the ion exchanger)enables the iodine compound to be removed. The contact of the aceticacid stream with the ion exchanger (or the running or passing of theacetic acid stream through the ion exchanger), if necessary, underheating continuously or stepwise, achieves prevention of the metal fromflowing out of the ion exchanger and effective removal of the iodinecompound. The ion exchange column may include a packed column having atleast an ion exchanger (e.g., a metal-supported ion exchanger) packedtherein, and a column provided with an ion exchanger bed (e.g., a bedhaving a granular ion exchanger) (a guard bed). The ion exchange columnmay be filled or packed with another ion exchanger (e.g., a cationexchange resin, an anion exchange resin, and a nonion exchange resin) inaddition to the ion exchanger. Moreover, the acetic acid stream may besubjected to the ion exchange treatment with a column filled or packedwith the ion exchanger and a column filled or packed with another ionexchanger. For example, the treatment unit may be provided with theanion exchange resin column and an ion exchange column containing ametal-supported ion exchange resin; the ion exchange column may belocated in a downstream side of the anion exchange resin or in anupstream side thereof. The details of the former embodiment may bereferred to, for example, WO02/062740.

The temperature of the ion exchange treatment may be a temperature of,for example, about 18 to 100° C., preferably about 30 to 70° C., andmore preferably about 40 to 60° C. The flow rate of the acetic acidstream may be, for example, about 3 to 15 bed volume/h, preferably about5 to 12 bed volume/h, and more preferably about 6 to 10 bed volume/h fora removal column having a guard bed.

The purification section (purification step group or purification unitgroup) (4) may comprise at least one step selected from the groupconsisting of the dehydration step (5), the higher boiling componentremoving step (6), the purification step (purification distillationcolumn) (7), and the ion exchange step (8). The purification section (4)usually comprises at least the dehydration step (5) and the higherboiling component removing step (6). The ion exchange step (8) may becarried out after any step of the purification section (4), for example,the dehydration step (5) and/or the higher boiling component removingstep (6), or may be carried out between the higher boiling componentremoving step (6) and the purification step (purification distillationcolumn) (7).

(9) Separation Section (Step Group or Unit Group)

As described above, the first overhead (3A) from the first distillationstep (3) contains impurities and useful components, such as PRC's,methyl iodide, and methyl acetate. Thus, in the separation section (stepgroup or unit group) (9), at least acetaldehyde is separated from thefirst overhead (3A). In particular, in the separation section (9), thefirst overhead (3A) is separated into a stream rich in acetaldehyde anda stream rich in useful methyl iodide.

The separation section (9) may comprise the following steps: (10) a stepfor condensing the first overhead (3A) to form two liquid phases withupper and lower phases (a liquid-liquid separation step), (11) a step (afirst aldehyde separation step or distillation step) for forming a fifthoverhead rich in acetaldehyde and methyl iodide from the upper phaseand/or the lower phase, (12) a step (an extraction step) for extractingacetaldehyde from the fifth overhead obtained in the step (11) to forman extract rich in acetaldehyde and a raffinate rich in methyl iodide,(13) a step(a second aldehyde separation step or distillation step) forseparating an aldehyde from the extract and/or the raffinate, and (14) astep (an alkane separation step or distillation step) for separating analkane from the upper phase and/or the lower phase.

(10) Liquid-Liquid Separation Step

In the liquid-liquid (or biphasic) separation step (10), the firstoverhead (3A) (the line 31) is cooled and condensed in a condenser toforma condensate 32 being rich in methyl iodide and containing water orother compounds, and the condensate 32 is separated into two phases, anaqueous phase 38 and an organic phase 39, in the decanter S2. A portionof the condensate (upper phase) is returned to the splitter column (3)for reflux via a reflux line 42, at least a portion of the upper phase(aqueous phase or light phase rich in acetaldehyde) 38 separated in thedecanter S2 is recycled to the reactor (1) via a line 41, and at least aportion of the lower phase (organic phase or heavy phase rich in methyliodide) 39 separated in the decanter S2 is recycled to the reactor (1)via a line 40.

Moreover, at least a portion of the lower phase (organic phase or heavyphase) rich in methyl iodide separated in the decanter S2 is fed to thefifth distillation column (distillation step) (11) via feed lines 111,112 for forming the fifth overhead rich in acetaldehyde and methyliodide. The lower phase (organic phase) rich in methyl iodide from thedecanter S2 is mixed with a portion (branched stream) 124 of a lowerstream (11B) (a line 123) from the fifth distillation column(distillation step) (11), and the mixture is recycled to the reactor(1).

A noncondensable gas (off-gas) 33 that has not been condensed in thecondenser is rich in methyl iodide and contains carbon monoxide or othercompounds. In the same manner as in the off-gas, the noncondensable gas(off-gas) 33 is fed to the off-gas treatment section (15) together witha noncondensable gas in the decanter S2 via lines 34, 35, 37, andtreated in the off-gas treatment section (15). The noncondensable gas(off-gas) is further cooled and condensed in a condenser on the line 34to form a condensate and a noncondensable gas; the condensate passesthrough a line 36 and is mixed with the lower phase (organic phase orheavy phase) 39 from a feed line 111, and the noncondensable gas is fedto a decanter S3 via the line 35. A condensate liquefied in the decanterS3 is mixed or merged with the lower phase (organic phase or heavyphase) 39 from the feed line 112, and a noncondensable gas in thedecanter S3 passes through the line 37 and is treated in the off-gastreatment section (15).

For example, the composition of the condensate 32 may be substantiallythe same as (or similar to) that of the first overhead (3A) (the line31). The composition of the condensate 32 may have a component ratio(content of each component) obtained calculated by subtracting thecomponent ratio of the noncondensable gas (off-gas) 33 that is notcondensed in the condenser from the component ratio of the firstoverhead (3A).

For example, the upper phase (aqueous phase) 38 in the decanter S2 mayhave the following composition.

TABLE 29 Average molecular weight 23.26 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%2 ppm to 0.1% CO 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to0.05% CO₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.05%CH₄ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% N₂ 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% AD 0 to 2% (e.g., 0.01to 2%) 0.05 to 1% 0.1 to 0.7% MeOH 0 to 10% (e.g., 10 ppm to 10%) 100ppm to 5% 0.1 to 3% MeI 0.1 to 15% 1 to 10% 2 to 6% MA 1 to 40% 5 to 30%10 to 25% H₂O 10 to 95% 20 to 90% 40 to 80% AcOH 1 to 30% 3 to 20% 8 to15% HI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.5% 1 ppm to 0.1% FrOH0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1% PrOH 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 50 ppm DME 0 to 1% (e.g., 0.1ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 1 ppmto 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EtOH 0 to 1% (e.g., 1 ppm to0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EA 0 to 1% (e.g., 1 ppm to 0.5%) 10ppm to 0.2% 50 ppm to 0.1% EtI 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to0.2% 50 ppm to 0.1% Li 0 to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1 ppt to0.01 ppm 1 ppt to 0.001 ppm Rh 0 to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1ppt to 0.01 ppm 1 ppt to 0.001 ppm

The composition of the upper phase (aqueous phase) in the line 41connecting to the reactor and splitter column (3) may also besubstantially the same as (or similar to) that of the upper phase(aqueous phase) 38 in the decanter S2.

For example, the lower phase (organic phase) 39 in the decanter S2 mayhave the following composition.

TABLE 30 Average molecular weight 119.79 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%2 ppm to 0.1% CO 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to0.05% CO₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.05%CH₄ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% N₂ 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% AD 0 to 2% (e.g., 0.01to 2%) 0.05 to 1% 0.08 to 0.5% MeOH 0 to 2% (e.g., 1 ppm to 2%) 10 ppmto 1% 100 ppm to 0.3% MeI 20 to 99% 40 to 95% 60 to 92% MA 1 to 40% 4 to30% 7 to 20% H₂O 0.01 to 20% 0.1 to 10% 0.5 to 3% AcOH 0.01 to 20% 0.1to 10% 0.5 to 3% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm 1 to100 ppm FrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 50 ppmPrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 30 ppm DME 0 to1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% (CH₃)₂C═O 0 to 1%(e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EtOH 0 to 1% (e.g.,1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EA 0 to 1% (e.g., 1 ppm to0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EtI 0 to 1% (e.g., 1 ppm to 0.5%) 10ppm to 0.2% 50 ppm to 0.1% Li 0 to 0.1% (e.g., 0.01 ppt to 0.1 ppm) 0.1ppt to 0.01 ppm 1 ppt to 0.001 ppm Rh 0 to 0.1% (e.g., 0.01 ppt to 0.1ppm) 0.1 ppt to 0.01 ppm 1 ppt to 0.001 ppm

For example, the noncondensable gas (off-gas) (the line 33) from thecondenser may have the following composition.

TABLE 31 Average molecular weight 79.89 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 1 ppm to 5% 5 ppm to 2.5% 50 ppm to 1% (e.g., 10 ppm to 1%)(e.g., 100 ppm to 0.1%) CO 0.01 to 50% 0.1 to 30% 1 to 15% CO₂ 0.01 to20% 0.1 to 10% 0.5 to 5% CH₄ 1 ppm to 10% 10 ppm to 3% 100 ppm to 1% N₂1 ppm to 10% 10 ppm to 3% 100 ppm to 1% AD 1 ppm to 10% 10 ppm to 3% 100ppm to 1% MeOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to0.1% MeI 1 to 99% 10 to 95% 40 to 90% MA 0.01 to 50% 0.1 to 20% 1 to 10%H₂O 0.001 to 20% 0.01 to 10% 0.1 to 5% AcOH 0.001 to 20% 0.01 to 10% 0.1to 5% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001%FrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001%PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% DME0 to 10% (e.g., 0.1 ppm to 10%) 1 ppm to 2% 10 ppm to 1% (CH₃)₂C═O 0 to1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g.,0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% Li 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb

For example, the noncondensable gas of the line 34 may have thefollowing composition.

TABLE 32 Average molecular weight 82.08 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 1 ppm to 5% 10 ppm to 1% 100 ppm to 0.1% CO 0.01 to 50% 0.1 to30% 1 to 15% CO₂ 0.01 to 20% 0.1 to 10% 0.5 to 5% CH₄ 1 ppm to 10% 10ppm to 3% 100 ppm to 1% N₂ 1 ppm to 10% 10 ppm to 3% 100 ppm to 1% AD 1ppm to 10% 10 ppm to 3% 100 ppm to 1% MeOH 0 to 1% (e.g., 0.1 ppm to 1%)1 ppm to 0.5% 10 ppm to 0.1% MeI 1 to 99% 10 to 95% 40 to 90% MA 0.01 to50% 0.1 to 20% 1 to 20% H₂O 0.001 to 20% 0.01 to 10% 0.1 to 5% AcOH0.001 to 20% 0.01 to 10% 0.1 to 5% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10ppb to 0.01% 100 ppb to 0.001% FrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppbto 0.01% 100 ppb to 0.001% PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% DME 0 to 10% (e.g., 0.1 ppm to 10%) 1 ppm to 2%10 ppm to 1% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppmto 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05%Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to1 ppb

For example, the noncondensable gas of the line 35 may have thefollowing composition.

TABLE 33 Average molecular weight 42.11 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 1 ppm to 10% 10 ppm to 5% 100 ppm to 1% CO 0.01 to 90% 0.1 to70% 1 to 50% CO₂ 0.01 to 30% 0.1 to 20% 0.5 to 10% CH₄ 10 ppm to 20% 100ppm to 10% 0.1% to 5% N₂ 10 ppm to 20% 100 ppm to 10% 0.1% to 5% AD 1ppm to 10% 10 ppm to 3% 100 ppm to 1% MeOH 0 to 1% (e.g., 0.1 ppm to 1%)1 ppm to 0.5% 10 ppm to 0.1% MeI 1 to 99% 10 to 90% 20 to 70% MA 0.01 to50% 0.1 to 20% 1 to 10% H₂O 0.001 to 20% 0.01 to 10% 0.1 to 5% AcOH 0 to10% (e.g., 1 ppm to 10%) 10 ppm to 5% 100 ppm to 1% HI 0 to 1% (e.g., 1ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% FrOH 0 to 1% (e.g., 1 ppbto 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% PrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 0.01% 100 ppb to 0.001% DME 0 to 10% (e.g., 0.1 ppm to10%) 1 ppm to 2% 10 ppm to 1% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to 1%)0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1ppm to 0.05% Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10ppb 1 ppt to 1 ppb

For example, the noncondensable gas of the line 36 may have thefollowing composition.

TABLE 34 Average molecular weight 112.80 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.01% CO 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100ppm to 0.1% CO₂ 1 ppm to 1% 10 ppm to 0.5% 100 ppm to 0.2% CH₄ 0 to 1%(e.g., 0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% N₂ 0 to 1% (e.g.,0.1 ppm to 0.1%) 1 ppm to 0.05% 10 ppm to 0.01% AD 100 ppm to 3% 0.1% to1% 0.2% to 6000 ppm MeOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100ppm to 0.3% MeI 20 to 99% 50 to 97% 70 to 90% MA 1 to 40% 5 to 30% 7 to20% H₂O 0.01 to 10% 0.1 to 8% 0.5 to 5% AcOH 0.001 to 5% 0.01 to 1% 0.05to 0.5% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm 1 to 100 ppmFrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 50 ppm PrOH 0 to1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 30 ppm DME 0 to 3% (e.g., 1ppm to 3%) 10 ppm to 2% 100 ppm to 1% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppmto 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1ppm to 0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.05% Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to10 ppb 1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 pptto 10 ppb 1 ppt to 1 ppb

For example, the noncondensable gas of the line 37 from the decanter S3may have the following composition.

TABLE 35 Average molecular weight 42.11 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0.01 to 5% 0.05 to 2% 0.1 to 1% CO 1 to 99% 5 to 80% 10 to 70%CO₂ 0.1 to 20% 0.5 to 15% 1 to 15% CH₄ 0.1 to 20% 0.5 to 15% 1 to 10% N₂0.1 to 20% 0.5 to 15% 1 to 10% AD 0 to 10% (e.g., 0.001 to 10%) 0.01 to5% 0.1 to 3% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to0.1% MeI 1 to 95% 10 to 90% 20 to 80% MA 0.01 to 40% 0.1 to 20% 1 to 10%H₂O 0 to 10% (e.g., 0.01 to 10%) 0.02 to 5% 0.05 to 2% AcOH 0 to 1%(e.g., 0.001 to 1%) 5 ppm to 0.5% 10 ppm to 0.1% HI 0 to 1% (e.g., 1 pptto 0.1%) 100 ppt to 0.01% 10 ppb to 1 ppm FrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 100 ppm 100 ppb to 10 ppm PrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 100 ppm 100 ppb to 10 ppm DME 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to 1%)0.1 ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppmto 0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.05% Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to10 ppb 1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 pptto 10 ppb 1 ppt to 1 ppb

For example, the composition of the feed line 111 of the lower phase(organic phase or heavy phase) may be substantially the same as (orsimilar to) the composition of the lower phase (organic phase) 39 in thedecanter S2.

For example, the composition of the feed line 112 may be substantiallythe same as (or similar to) the composition of the lower phase (organicphase) 39 in the decanter S2. Moreover, the composition of the feed line112 may have a component ratio (content of each component) obtained orcalculated by the weighted average of the component ratio of the feedline 111 of the lower phase (organic phase or heavy phase) and thecomponent ratio of the line 36 of the condensate.

The ratio (weight ratio) of the flow rate of the upper phase withdrawnfrom the decanter S2 relative to that of the lower phase withdrawn fromthe decanter S2 [the upper phase/the lower phase] may be, for example,about 0.1/1 to 10/1 (e.g., about 0.3/1 to 3/1) and preferably about0.5/1 to 2/1 (e.g., about 0.7/1 to 1.5/1). The ratio (weight ratio) ofthe flow rate of the upper phase refluxed to the splitter column (3)relative to that of the upper phase recycled to the reaction system (1)[the former/the latter] may be about 2/1 to 1000/1 (e.g., about 5/1 to200/1) and preferably about 10/1 to 100/1 (e.g., about 15/1 to 50/1).

The first overhead (3A) may be fed to the first aldehyde separation step(11) without condensation or liquid-liquid separation in the decanterS2.

Alternatively, the first overhead (3A) may biphasically be separatedinto an upper phase (an aqueous phase or light phase) rich inacetaldehyde and a lower phase (an organic phase or heavy phase) rich inmethyl iodide in the decanter, and at least one phase of the upper phaseand the lower phase may be fed to the first aldehyde separation step(11) and/or the reactor (1). Moreover, to the reactor (1), a portion ofthe upper phase (aqueous phase or light phase) is recycled, or,alternatively, the lower phase (organic phase or heavy phase) may berecycled. Further, to the first aldehyde separation step (11), the upperphase (aqueous phase or light phase) may be fed instead of the lowerphase (organic phase or heavy phase).

In a case where the first overhead (3A) is cooled successively insequentially arranged plural condensers successively lower in coolingtemperature to form a plurality of condensates successively lower intemperature, a condensate formed by a subsequent condenser has a higherconcentration of acetaldehyde compared with a process liquid (acondensate) formed by a first condenser. Accordingly, a condensatehaving a high concentration of acetaldehyde may be fed to the fourthdistillation step (11) to separate acetaldehyde from the condensate.

(11) First Aldehyde Separation Step (Fifth Distillation Step)

The condensate of the line 112 from the decanter S2 in the liquid-liquidseparation step (10) [at least the portion 112 of the organic phase orheavy phase 39 being rich in methyl iodide and containing methyl acetateor other compounds] is heated by a heating unit and is then held in ahold tank S4 for gas-liquid separation or degasification treatment toform a condensation mixture (being rich in methyl iodide and containingmethyl acetate or other compounds) and a gaseous phase. The condensationmixture from the hold tank S4 is fed, for distillation, to the firstaldehyde separation step (fifth distillation step or distillationcolumn) (11) via a line 114 to form a fifth overhead (11A) rich inacetaldehyde and methyl iodide. Specifically, in this embodiment, theorganic phase (heavy phase rich in methyl iodide) formed in theliquid-liquid separation step (10) is fed to the first aldehydeseparation step (distillation step or distillation column) (11) via thefeed lines 112, 114, and is distilled to forma fifth overhead (11A) anda lower stream (11B); the fifth overhead (11A) is withdrawn from a topor upper part of the column via a withdrawing line (a line 115), and thelower stream (11B) is withdrawn via a bottom line 123. The gaseous phasefrom the hold tank S4, which contains methyl iodide or other compounds,is mixed, through a line 113, with the noncondensable gas from thedecanter S2 of the liquid-liquid separation step (10), and the mixtureis cooled and condensed in a condenser.

The lower stream (11B) 123 contains acetic acid, water, or othercompounds. A first portion of the lower stream (11B) is recycled to thefirst aldehyde separation step (distillation column) (11); a secondportion (or residual portion) of the lower stream (11B) is mixed,through a line 124, with the lower phase (organic phase), rich in methyliodide, from the decanter S2, and the resulting mixture is recycled tothe reactor (1).

The fifth overhead (11A) (the line 115) is rich in acetaldehyde andmethyl iodide. The fifth overhead (11A) is cooled and condensed incondensers to form condensates and a noncondensable gases. Thecondensates are sent to a hold tank T6 for holding or storage via lines117, 119. The noncondensable gases, which contain methyl iodide or othercompounds, are mixed, through lines 116, 118, with the noncondensablegas from the decanter S2 of the liquid-liquid separation step (10), andthe mixture is cooled and condensed in a condenser. Moreover, a portionof the condensate from the hold tank T6 is returned to the firstaldehyde separation step (distillation step) (11) via lines 120, 121 forreflux, and the residual portion of the condensate is cooled in acondenser on a line 122 and is fed to a water extraction unit (waterextractive distillation column) (12) of the extraction step (12) via aline 125.

For example, the condensate (the line 114) from the decanter S4 may havethe following composition.

Incidentally, the composition of the condensate (the line 114) from thedecanter S4 may be substantially the same as (or similar to) thecomposition of the organic phase 39 in the decanter S2 or thecomposition of the feed line 112, except for the concentrations ofcomponents in the noncondensable gas and the concentration of DME. Thus,the following table shows the concentrations of components in thenoncondensable gas and the concentration of DME.

TABLE 36 Average molecular weight 119.03 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, 10%), e.g., 0.2 ppb to 3.6% ppt to 1000 ppm), e.g.,100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 ppm to 0.01% 2 to500 ppm CO 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 ppm to 0.01% 1 to 500 ppmCO₂ 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 ppm to 0.01% 1 to 500 ppm CH₄ 0 to1% (e.g., 0.1 ppm to 0.1%) 1 ppm to 0.01% 1 to 500 ppm N₂ 0 to 1% (e.g.,0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% DME 0 to 1% (e.g., 0.01 ppmto 0.1%) 0.05 to 100 ppm 0.5 to 20 ppm

For example, the noncondensable gas (the line 113) from the decanter S4may have the following composition.

TABLE 37 Average molecular weight 100.95 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 5% (e.g., 1 ppm to 5%) 10 ppm to 1% 100 ppm to 0.1% CO0.001 to 10% 0.01 to 5% 0.1 to 1% CO₂ 0.001 to 20% 0.01 to 10% 0.1 to 2%CH₄ 0 to 5% (e.g., 1 ppm to 5%) 10 ppm to 1% 100 ppm to 0.1% N₂ 0 to 5%(e.g., 1 ppm to 5%) 10 ppm to 1% 100 ppm to 0.1% AD 0.001 to 10% 0.01 to5% 0.1 to 1% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to0.1% MeI 1 to 99% 10 to 95% 40 to 90% MA 0.01 to 50% 0.1 to 20% 1 to 10%H₂O 0.001 to 20% 0.01 to 10% 0.1 to 5% AcOH 0 to 2% (e.g., 0.001 to 2%)0.005 to 1% 0.01 to 0.5% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% FrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% DME 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1%1 ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppmto 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05%Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to1 ppb

For example, the fifth overhead (11A) (the line 115) may have thefollowing composition.

TABLE 38 Average molecular weight 71.30 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.01%CO 0 to 3% (e.g., 0.1 ppm to 3%) 1 ppm to 1% 10 ppm to 0.1% CO₂ 0 to 3%(e.g., 0.1 ppm to 3%) 1 ppm to 1% 10 ppm to 0.1% CH₄ 0 to 1% (e.g., 0.01ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.01% N₂ 0 to 1% (e.g., 0.01 ppm to1%) 0.1 ppm to 0.1% 1 ppm to 0.01% AD 5 to 90% (e.g., 10 to 80%) 15 to75% 20 to 60% MeOH 0 to 5% (e.g., 0.1 ppm to 5%) 1 ppm to 1% 10 ppm to0.1% MeI 5 to 95% (e.g., 10 to 90%) 20 to 85% 40 to 80% MA 0.1 ppm to 5%1 ppm to 1% 10 ppm to 0.5% H₂O 1 ppm to 10% 10 ppm to 2% 100 ppm to 1%AcOH 0 to 1% (e.g., 1 ppb to 1%) 10 ppb to 0.1% 100 ppb to 0.01% HI 0 to1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% FrOH 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% PrOH 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% DME 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g.,0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% Li 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb

For example, the condensates (lines 120, 121, 122) of the fifth overhead(the line 115) from the condenser (s) may have the followingcomposition.

Incidentally, the composition of the condensates (lines 120, 121, 122)may be substantially the same as (or similar to) the composition of thefifth overhead (11A) (the line 115), except for the concentrations ofcomponents in the noncondensable gas. Thus, the following table showsthe concentrations of components in the noncondensable gas.

TABLE 39 Average molecular weight 71.33 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, e.g., 10%), e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),100 ppt to 100 ppm (e.g., 1 ppb to 2%) e.g., less than 700 ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppbto 0.001% CO 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to0.01% CO₂ 0 to 2% (e.g., 0.1 ppm to 2%) 1 ppm to 1% 10 ppm to 0.5% CH₄ 0to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.01% N₂ 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.01%

For example, the noncondensable gas (line 118) of the fifth overhead(the line 115) from the condenser(s) may have the following composition.

As described above, an inactive gas (nitrogen gas, carbon monoxide gas)purge may be introduced in order to control the pressure of adistillation column and to protect a level gauge, a pressure gauge, athermometer, or other measuring instruments, from a condensable gas. Thecomposition of the noncondensable gas in the lines 115, 116, 118significantly changes due to the introduction of such a component, andthe concentrations of other components also significantly change due tothe dilution with the introduced inactive gas.

TABLE 40 Average molecular weight 39.25 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% CO0.01 to 20% 0.1 to 15% 0.5 to 10% CO₂ 0.01 to 70% 0.1 to 60% 0.5 to 50%CH₄ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% N₂ 10ppm to 70% 100 ppm to 50% 0.1% to 40% AD 1 ppm to 30% 10 ppm to 25% 100ppm to 20% MeOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to0.1% MeI 1 to 50% 2 to 40% 5 to 25% MA 0 to 10% (e.g., 5 ppm to 1%) 10ppm to 0.5% 25 ppm to 0.1% H₂O 0 to 10% (e.g., 10 ppm to 5%) 50 ppm to1% 50 ppm to 0.1% AcOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.3% 100ppm to 0.1% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.001% FrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.001% PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to0.001% DME 0 to 2% (e.g., 0.1 ppm to 1%) 1 ppm to 1% 10 ppm to 0.1%(CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05%EtOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EA 0to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% EtI 0 to 1%(e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 ppm to 0.05% Li 0 to 0.1%(e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb Rh 0 to0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb

For example, the lower streams (11B) 123, 124 of the fifth distillationcolumn (11) may have the following composition.

TABLE 41 Average molecular weight 119.33 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to0.01% 100 ppt to 0.001% CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01%100 ppt to 0.001% CO₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100ppt to 0.001% CH₄ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 pptto 0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% AD 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 100 to 700 ppmMeOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 ppm to 0.01% MeI 1to 99% (e.g., 10 to 95%) 30 to 98% 70 to 90% (e.g., 50 to 95%) MA 1 to40% 5 to 30% 7 to 20% H₂O 0.01 to 30% 0.1 to 10% 0.5 to 5% AcOH 0.01 to10% 0.1 to 5% 0.5 to 3% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.05 ppm to0.2% 0.5 ppm to 0.1% (e.g., 0.1 to 500 ppm) (e.g., 1 to 100 ppm) FrOH 0to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm PrOH 0to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm DME 0to 1% (e.g., 1 ppb to 100 ppm) 5 ppb to 0.5% 50 ppb to 0.1% (e.g., 10ppb to 10 ppm) (e.g., 100 ppb to 5 ppm) (CH₃)₂C═O 0 to 1% (e.g., 1 ppbto 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm EtOH 0 to 1% (e.g., 1 ppbto 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm EA 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm EtI 0 to 1% (e.g., 1 ppb to100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm Li 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 pptto 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb

In the first aldehyde separation step (distillation step or distillationcolumn) (11), the condensate formed in the liquid-liquid separation step(10) is distilled to form a fifth overhead (11A) rich in acetaldehydeand methyl iodide. The upper phase (aqueous phase, light phase rich inacetaldehyde) may be distilled, the lower phase (organic phase, heavyphase rich in methyl iodide) may be distilled, or the condensationmixture of the upper phase and the lower phase may be distilled. As thedistillation column of the fifth distillation step (11), there may beused a plate column, a packed column, or other columns.

(12) Extraction Step (Extractive Distillation Column or SixthDistillation Column)

In the water extraction unit (water extractive distillation column) ofthe extraction step (12), acetaldehyde is extracted from the fifthoverhead (11A) (a condensate cooled in a condenser), forming an extractrich in acetaldehyde and a raffinate rich in methyl iodide.Specifically, to the extraction step (extractive distillation column orsixth distillation column) (12), the condensate and an extractant(water) are fed from a feed line 125 and a lower feed line 126,respectively. In the extraction step (water extraction column) (12), thefifth overhead (11A) is separated into a water extract (an extractcontaining acetaldehyde) (12A) withdrawn from the column top or via anupper withdrawing line 131, and a raffinate (12B) rich in methyl iodidewithdrawn via a bottom line 132. The raffinate 132 may be discharged asa waste fluid or may be recycled to the reactor (1). The water extract131 is further fed to the second aldehyde separation step (distillationstep or distillation column) (13).

For example, the extractant (water) from the feed line 126 may have thefollowing composition.

TABLE 42 Average molecular weight 18.02 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppt to 1%) 10 ppt to 0.1%100 ppt to 0.01% CO 0 to 1% (e.g., 1 ppt to 1%) 10 ppt to 0.1% 100 pptto 0.01% CO₂ 0 to 1% (e.g., 1 ppt to 1%) 10 ppt to 0.1% 100 ppt to 0.01%CH₄ 0 to 1% (e.g., 1 ppt to 1%) 10 ppt to 0.1% 100 ppt to 0.01% N₂ 0 to1% (e.g., 1 ppt to 1%) 10 ppt to 0.1% 100 ppt to 0.01% AD 0 to 0.01%(e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm MeOH 0 to 0.01%(e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm MeI 0 to 0.01%(e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm MA 0 to 0.01%(e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1 to 10 ppm H₂O 99 to 100%99.5 to 99.999% 99.9 to 99.99% AcOH 0 to 1% (e.g., 0 to 0.01%) 0.01 to50 ppm 0.1 to 10 ppm HI 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50ppm 0.1 to 10 ppm FrOH 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50ppm 0.1 to 10 ppm PrOH 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50ppm 0.1 to 10 ppm DME 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50 ppm0.1 to 10 ppm (CH₃)₂C═O 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50ppm 0.1 to 10 ppm EtOH 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50ppm 0.1 to 10 ppm EA 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50 ppm0.1 to 10 ppm EtI 0 to 0.01% (e.g., 0.001 to 100 ppm) 0.01 to 50 ppm 0.1to 10 ppm Li 0 to 0.1% (e.g., 0 to 1 ppm) 1 ppt to 10 ppb 10 ppt to 1ppb Rh 0 to 0.1% (e.g., 0 to 1 ppm) 1 ppt to 10 ppb 10 ppt to 1 ppb

For example, the water extract 131 may have the following composition.

TABLE 43 Average molecular weight 21.43 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0.1% or less (e.g., 1 ppt to 0.1%) 10 ppt to0.01% 100 ppt to 0.001% CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01%100 ppt to 0.001% CO₂ 0 to 5% (e.g., 1 ppt to 5%) 10 ppt to 3% 100 pptto 1% CH₄ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.5%) 10 ppt to 0.1% 100 ppt to 0.01%AD 1 to 50% 3 to 40% 5 to 30% MeOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppmto 0.1% 10 ppm to 0.01% MeI 0 to 25% (e.g., 0.1 to 20%) 0.5 to 20% 1 to15% MA 0 to 2% (e.g., 10 ppb to 1%) 100 ppb to 1% 1 ppm to 0.5% H₂O 10to 98% 50 to 95% 60 to 90% AcOH 0 to 5% (e.g., 10 ppb to 1%) 100 ppb to3% 1 ppm to 1% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm 1 to100 ppm FrOH 0 to 1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10 ppbto 1 ppm PrOH 0 to 1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10 ppbto 1 ppm DME 0 to 1% (e.g., 10 ppb to 1%) 100 ppb to 0.2% 1 to 500 ppm(CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EtOH 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EA 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EtI 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 pptto 1 ppb

For example, the raffinate 132 may have the following composition.

TABLE 44 Average molecular weight 115.86 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to 10%), less than 1%(e.g., 1 10 ppt to 300 ppm, e.g., 0.2 ppb to 3.6% ppt to 1000 ppm),e.g., 100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm(e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to0.01% 100 ppt to 0.001% CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01%100 ppt to 0.001% CO₂ 0 to 5% (e.g., 1 ppt to 5%) 10 ppt to 2% 100 pptto 1% CH₄ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.5%) 10 ppt to 0.1% 100 ppt to 0.01%AD 0.1 to 30% 1 to 20% 5 to 15% MeOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1ppm to 0.1% 1 to 100 ppm MeI 80 to 100% 90 to 99.999% 99 to 99.99% MA0.1 ppm to 2% 1 ppm to 1% 10 ppm to 0.5% H₂O 10 ppm to 2% 100 ppm to 1%500 ppm to 0.5% AcOH 0 to 1% (e.g., 10 ppb to 1%) 100 ppb to 0.5% 1 ppmto 0.1% HI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 500 ppm 100 ppb to100 ppm FrOH 0 to 1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10 ppbto 1 ppm PrOH 0 to 1% (e.g., 0.1 ppb to 100 ppm) 1 ppb to 10 ppm 10 ppbto 1 ppm DME 0 to 1% (e.g., 10 ppb to 1%) 100 ppb to 0.2% 1 to 500 ppm(CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EtOH 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EA 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm EtI 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 ppt to1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 0.1 ppt to 10 ppb 1 pptto 1 ppb

For the water extractive distillation, the condensation mixture is fedto the water extractive distillation column, and an extractant (water)is fed to an upper part of the distillation column for forming araffinate rich in methyl iodide as an overhead and a water extract (anextract containing acetaldehyde) as a bottom liquid. As the waterextractive distillation column, there may be used a plate column, apacked column, or other columns.

In the extraction step (12), an extraction unit (extractor) may be usedinstead of the extractive distillation. The extraction unit may compriseone or a plurality of extractors. As each one of the extractors, forexample, there may be used a combination of a mixer with a settler, acombination of a static mixer with a decanter, a rotated disk contactor(RDC), a Karr column, a spray column, a packed column, a perforatedplate column, a baffled column, and a pulse column. The extractor(extraction column) may be a single-stage extraction unit for extractingan object from a mixture of the object and water and separating themixture into liquid phases, or may have a plurality of the single-stageextraction units arranged in a cascade manner. For example, theextractor may be a multi-stage extraction unit that comprises aplurality of extractors (each extractor having a theoretical number ofplates of 1) for sequential extraction. Moreover, the extractor may be amulti-stage extraction unit in which a plurality of extractors has beeninstalled in a single unit, for example, a single extraction unit havingthe theoretical number of plates equivalent to a multi-stage extractionunit (the theoretical number of plates corresponding to multi-stageextraction). Moreover, the extraction may be either a batch system or acontinuous system, or may be performed in either a parallel extractionor a countercurrent extraction.

Further, at least a portion of the raffinate may be recycled to thereactor, or at least a portion of the water extract may be fed to thesucceeding second aldehyde separation step (distillation step ordistillation column) (13).

(13) Second Aldehyde Separation Step (Seventh Distillation Column)

In the second aldehyde separation step (seventh distillation step ordistillation column) (13), the water extract 131 rich in acetaldehyde isdistilled to give a sixth overhead (13A) withdrawn from a top or upperpart of the column via a withdrawing line 141 and a lower stream (13B)withdrawn via a bottom line 146, in the same manner as in the firstaldehyde separation step (distillation step or distillation column)(11).

The lower stream (13B) (the line 146) contains water or other compounds.A first portion of the lower stream (13B) is recycled to the secondaldehyde separation step (distillation column) (13), and a secondportion (or residual portion) of the lower stream (13B) is sent to anincinerator via a line 146 for incineration.

The sixth overhead (13A), which is rich in acetaldehyde, is cooled andcondensed in condenser on the withdrawing line 141 to form a condensateand a noncondensable gas. The condensate is held in a hold tank T7 via aline 142, a portion of the condensate is returned to the second aldehydeseparation step (distillation step) (13) for reflux via a reflux line143, and the residual portion of the condensate is incinerated by anincineration unit via a line 144. Moreover, the noncondensable gas thatis not condensed in the condenser is mixed, through a line 145, with thenoncondensable gas from the decanter S2 of the liquid-liquid separationstep (10), and the mixture is cooled and condensed in a condenser.

For example, the sixth overhead (13A) (the line 141) may have thefollowing composition.

TABLE 45 Average molecular weight 42.08 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001%CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO₂ 0to 1% (e.g., 1 ppt to 1%) 10 ppt to 0.5% 100 ppt to 0.05% CH₄ 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% N₂ 0 to 5%(e.g., 1 ppt to 5%) 10 ppt to 1% 100 ppt to 0.1% AD 50 to 99% 70 to 95%80 to 90% MeOH 0 to 5% (e.g., 0.1 ppm to 5%) 1 ppm to 1% 10 ppm to 0.1%MeI 0.1 to 30% 0.5 to 20% 1 to 10% MA 0.1 ppm to 3% 1 ppm to 1% 10 ppmto 0.5% H₂O 0.1 to 30% 1 to 20% 3 to 10% AcOH 0 to 1% (e.g., 0.1 ppb to0.1%) 1 ppb to 0.01% 10 ppb to 0.001% HI 0 to 1% (e.g., 1 ppb to 0.5%)10 ppb to 0.1% 100 ppb to 200 ppm FrOH 0 to 1% (e.g., 0.1 ppb to 100ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm PrOH 0 to 1% (e.g., 0.1 ppb to 100ppm) 1 ppb to 10 ppm 10 ppb to 1 ppm DME 0 to 1% (e.g., 10 ppb to 1%)100 ppb to 0.2% 1 to 500 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 100 ppm)10 ppb to 10 ppm 100 ppb to 5 ppm EtOH 0 to 1% (e.g., 1 ppb to 100 ppm)10 ppb to 10 ppm 100 ppb to 5 ppm EA 0 to 1% (e.g., 1 ppb to 100 ppm) 10ppb to 10 ppm 100 ppb to 5 ppm EtI 0 to 1% (e.g., 1 ppb to 100 ppm) 10ppb to 10 ppm 100 ppb to 5 ppm Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb)0.1 ppt to 10 ppb 1 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100ppb) 0.1 ppt to 10 ppb 1 ppt to 1 ppb

The composition of the condensates 143, 144 from the condenser may besubstantially the same as (or similar to) the composition of the sixthoverhead (13A) (the line 141), except for the concentrations ofcomponents in the noncondensable gas. Thus, the following table showsthe concentrations of components in the noncondensable gas.

TABLE 46 Average molecular weight 42.08 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% (e.g., 500ppb to 500 ppm (e.g., 10%), e.g., 10 ppb to 3.6% 100 ppb to 0.1%), or 1to 100 ppm), or (e.g., 20 ppb to 2%), or less 20 ppb to 0.3% 50 ppb to0.1% than 7% (e.g., 1 ppt to 5%), (e.g., 100 ppb to 200 e.g., less than3.6% (e.g., 0.1 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001%CO 0 to 1% (e.g., 1 ppt to 0.01%) 10 ppt to 0.001% 100 ppt to 0.0001%CO₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.05% 100 ppt to 0.005% CH₄0 to 1% (e.g., 1 ppt to 0.01%) 10 ppt to 0.001% 100 ppt to 0.0001% N₂ 0to 1% (e.g., 1 ppt to 0.5%) 10 ppt to 0.1% 100 ppt to 0.01%

For example, the noncondensable gas 145 from the condenser may have thefollowing composition.

As described above, the composition of the noncondensable gas 145significantly changes according to the amount of the inactive purge gas.

TABLE 47 Average molecular weight 37.56 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% (e.g., 5%), e.g., less than 3.6% (e.g.,0.1 100 ppb to 200 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.01% CO0.01 to 60% 0.1 to 50% 0.5 to 40% CO₂ 0.01 to 60% 0.1 to 50% 0.5 to 40%CH₄ 0 to 3% (e.g., 0.1 ppm to 2%) 1 ppm to 1% 10 ppm to 0.5% N₂ 0 to 3%(e.g., 10 ppm to 2%) 50 ppm to 1% 100 ppm to 0.5% AD 0.1 to 90% 1 to 80%5 to 70% MeOH 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.5% 10 ppm to 0.1%MeI 100 ppm to 50% 0.1 to 20% 0.5 to 5% MA 0.001 to 10% 0.01 to 5% 0.1to 2% H₂O 0 to 1% (e.g., 0.0001 to 2%) 0.001 to 1% 0.01 to 0.1% AcOH 0to 1% (e.g., 0.0001 to 2%) 0.001 to 1% 0.01 to 0.1% HI 0 to 1% (e.g., 1ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% FrOH 0 to 1% (e.g., 1 ppbto 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% PrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 0.01% 100 ppb to 0.001% DME 0 to 1% (e.g., 0.1 ppm to10%) 1 ppm to 2% 10 ppm to 1% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppm to 1%)0.1 ppm to 0.1% 1 ppm to 0.05% EtOH 0 to 1% (e.g., 0.01 ppm to 1%) 0.1ppm to 0.1% 1 ppm to 0.05% EA 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to0.1% 1 ppm to 0.05% EtI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1ppm to 0.05% Li 0 to 0.1% (e.g., 0.1 ppt to 100 ppb) 1 ppt to 10 ppb 10ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.1 ppt to 100 ppb) 1 ppt to 10 ppb 10ppt to 1 ppb

For example, the lower stream (13B) (the line 146) may have thefollowing composition.

TABLE 48 Average molecular weight 18.02 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 10 pptto 300 ppm, ppb to 10%), e.g., 0.2 ppt to 1000 ppm), e.g., 100 ppt to100 ppb to 3.6% (e.g., e.g., less than 700 ppm 1 ppb to 2%) ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 pptto 0.001% CO 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% CO₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% CH₄ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% N₂ 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to0.001% AD 0 to 1% (e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 100 to 700 ppmMeOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 ppm to 0.01% MeI 0to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 ppm to 0.01% MA 0 to 1%(e.g., 0.1 ppm to 0.5%) 1 ppm to 0.1% 10 ppm to 0.01% H₂O 90 to 100% 98to 99.999% 99 to 99.99% AcOH 0 to 1% (e.g., 0.1 ppm to 0.5%) 1 ppm to0.1% 10 ppm to 0.01% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm1 to 100 ppm FrOH 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100ppb to 5 ppm PrOH 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100ppb to 5 ppm DME 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100ppb to 5 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm100 ppb to 5 ppm EtOH 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm100 ppb to 5 ppm EA 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm100 ppb to 5 ppm EtI 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm100 ppb to 5 ppm Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10ppb 10 ppt to 1 ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10ppb 10 ppt to 1 ppb

In the second aldehyde separation step (distillation step ordistillation column) (13), at least a portion of the raffinate 132 richin methyl iodide may be distilled instead of at least a portion of thewater extract 131; at least a portion of the water extract 131 and atleast a portion of the raffinate 132 may be distilled to form theoverhead (13A) containing acetaldehyde. As the distillation column ofthe fifth distillation step (13), there may be used a plate column, apacked column, or other columns.

(14) Alkane Separation Step (Distillation Step, Eighth DistillationColumn)

In the alkane separation step (distillation step) (14), an alkane isseparated from the portion 41 of the upper phase and/or the portion 40of the lower phase formed in the liquid-liquid separation step (10).Specifically, in this embodiment, the portion 40 of the lower phase(organic phase or heavy phase rich in methyl iodide) is distilled in thealkane separation step (distillation step) (14) to give a seventhoverhead (14A) withdrawn from a top or upper part of the column via awithdrawing line 151 and a lower stream (14B) withdrawn via a bottomline 152.

A portion of the lower stream (14B) containing an alkane may be heatedto be recycled to the alkane separation step (distillation step) (14),and the residual portion of the lower stream (14B) may be sent to anincinerator unit for incineration.

The seventh overhead (14A), which contains acetaldehyde and methyliodide, is cooled and condensed in a condenser on a withdrawing line 151to form a condensate and a noncondensable gas. The condensate is held ina tank T8, a portion of the condensate is returned to the alkaneseparation step (distillation step) (14) for reflux, and the residualportion of the condensate is recycled to the reactor (1). Thenoncondensable gas is mixed, through a line 113, with the noncondensablegas from the decanter S2 of the liquid-liquid separation step (10), andthe mixture is cooled and condensed in a condenser.

For example, the composition of the seventh overhead (14A) (the line151) may have substantially the same as (or similar to) the compositionof the organic phase 39 in the decanter S2.

For example, the lower stream (14B) (the line 152) may have thefollowing composition.

TABLE 49 Average molecular weight 119.79 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 10 pptto 300 ppm, ppb to 10%), e.g., 0.2 ppt to 1000 ppm), e.g., 100 ppt to100 ppb to 3.6% (e.g., 1 e.g., less than 700 ppm ppb to 2%) ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to0.1% CO 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.05% CO₂0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 10 ppm to 0.05% CH₄ 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% N₂ 0 to 1% (e.g., 0.1ppm to 1%) 1 ppm to 0.1% 2 ppm to 0.1% AD 0 to 2% (e.g., 0.01 to 2%)0.05 to 1% 0.08 to 0.5% MeOH 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1%100 ppm to 0.3% MeI 0 to 1% (e.g., 0.1 ppm to 5%) 1 ppm to 1% 10 ppm to0.1% Alkane 1 to 99% 10 to 80% 20 to 70% (C₄ to C₂₀) MA 1 to 40% 4 to30% 7 to 20% H₂O 0.01 to 20% 0.1 to 10% 0.5 to 3% AcOH 0.1 to 30% 0.3 to20% 0.5 to 15% HI 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 500 ppm 1 to100 ppm FrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 50 ppmPrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 100 ppm 3 to 30 ppm DME 0 to1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 100 ppm to 0.1% (CH₃)₂C═O 0 to 1%(e.g., 1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EtOH 0 to 1% (e.g.,1 ppm to 0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EA 0 to 1% (e.g., 1 ppm to0.5%) 10 ppm to 0.2% 50 ppm to 0.1% EtI 0 to 1% (e.g., 1 ppm to 0.5%) 10ppm to 0.2% 50 ppm to 0.1% Li 0 to 1% (e.g., 0.01 ppt to 0.1 ppm) 0.1ppt to 0.01 ppm 1 ppt to 0.001 ppm Rh 0 to 1% (e.g., 0.01 ppt to 0.1ppm) 0.1 ppt to 0.01 ppm 1 ppt to 0.001 ppm

As the distillation column of the eighth distillation step or alkaneseparation step (14), there may be used a plate column, a packed column,or other columns.

Among the above-mentioned steps (10) to (14), the separation section (9)usually comprises at least the liquid-liquid separation step (10), thefirst aldehyde separation step or distillation step (11), the extractionstep (12), and the first aldehyde separation step or distillation step(13).

(15) Off-Gas Treatment Section (or Step Group or Unit Group)

The off-gas produced from the process described above also containsuseful components such as carbon monoxide and methyl iodide. Thus,preferably, the off-gas is treated in the off-gas treatment section (15)to give the useful components which are then collected. The off-gastreatment section (15) may comprise, for example, the steps of: (16)absorbing the off-gas to the absorption solvent at a high pressure (ahigh-pressure absorption step), (17) absorbing the off-gas to theabsorption solvent at a low pressure (a low-pressure absorption step),and (18) diffusing a gaseous component absorbed in the absorption steps(16) and (17) (a diffusion step).

Examples of the absorption solvent to be used may include an aceticacid-based solvent and a methanol-based solvent. For example, the aceticacid-based solvent may have the following composition.

TABLE 50 Range Preferred range More preferred range O₂ 10% or less(e.g., 0.1 less than 1% (e.g., 1 10 ppt to 300 ppm, ppb to 10%), e.g.,0.2 ppt to 1000 ppm), e.g., 100 ppt to 100 ppb to 3.6% (e.g., e.g., lessthan 700 ppm 1 ppb to 2%) ppm (e.g., 1 ppt to 500 ppm) H₂ 0 to 1% (e.g.,1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% CO 0 to 1% (e.g., 1 ppbto 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% CO₂ 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 0.01% 100 ppb to 0.001% CH₄ 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 0.01% 100 ppb to 0.001% N₂ 0 to 1% (e.g., 1 ppb to 0.1%)10 ppb to 0.01% 100 ppb to 0.001% AD 0 to 1% (e.g., 1 ppb to 0.1%) 10ppb to 0.01% 100 ppb to 0.001% MeOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppbto 0.01% 100 ppb to 0.001% MeI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% MA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01%100 ppb to 0.001% H₂O 10 ppm to 5% 100 ppm to 2% 0.1 to 1% AcOH 80 to100% 90 to 99.99% 98 to 99.9% HI 0 to 1% (e.g., 0.1 ppb to 0.5%) 1 ppbto 0.1% 10 ppb to 100 ppm FrOH 0 to 1% (e.g., 1 to 300 ppm) 5 to 100 ppm10 to 50 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 200 ppm 10 to 100ppm DME 0 to 1% (e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5ppm AcA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to50 ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to50 ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm EtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm Li 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10 ppb 10 ppt to 1ppb Rh 0 to 0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10 ppb 10 ppt to 1ppb

For example, the methanol-based solvent may have the followingcomposition.

TABLE 51 Range Preferred range More preferred range O₂ 10% or less(e.g., 0.1 less than 1% (e.g., 1 10 ppt to 300 ppm, ppb to 10%), e.g.,0.2 ppt to 1000 ppm), e.g., 100 ppt to 100 ppb to 3.6% (e.g., 1 e.g.,less than 700 ppm ppb to 2%) ppm (e.g., 1 ppt to 500 ppm) H₂ 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CO₂ 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% CH₄ 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% N₂ 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 0.01% 100 ppt to 0.001% AD 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% MeOH 95 to 100%98 to 99.999% 99 to 99.99% MeI 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to0.01% 100 ppt to 0.001% MA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01%100 ppb to 0.001% H₂O 0 to 1% (e.g., 1 ppm to 0.1%) 10 ppm to 0.05% 100ppm to 0.01% AcOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppbto 0.001% HI 0 to 1% (e.g., 0.1 ppb to 0.5%) 1 ppb to 0.1% 10 ppb to 100ppm FrOH 0 to 1% (e.g., 1 to 300 ppm) 5 to 100 ppm 10 to 50 ppm PrOH 0to 1% (e.g., 0.1 ppm to 0.1%) 1 to 200 ppm 10 to 100 ppm DME 0 to 1%(e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm AcA 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm (CH₃)₂C═O 0 to1% (e.g., 1 ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 30 ppm EtOH 0 to1% (e.g., 1 ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm EA 0 to1% (e.g., 1 ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm EtI 0 to1% (e.g., 1 ppb to 100 ppm) 10 ppb to 50 ppm 100 ppb to 10 ppm Li 0 to0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb Rh 0 to0.1% (e.g., 0.01 ppt to 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb Fe 0 to0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.5 ppm 100 ppt to 0.1 ppm

Incidentally, methyl iodide MeI, methyl acetate MA, acetic acid AcOH,hydrogen iodide HI, formic acid FrOH, propionic acid PrOH, aceticanhydride AcA, lithium Li, and rhodium Rh are not usually detected fromthe methanol-based solvent.

(16) High-Pressure Absorption Step

In the off-gas treatment section (15), the noncondensable gas (theoff-gas rich in carbon monoxide and methyl iodide) 11 from the reactor(1) is put into contact with an acetic acid 197 as an absorption solventin the high-pressure absorption column (16) of the high-pressureabsorption step or first absorption step (16), and the mixture isscrubbed to form an overhead stream 171 and a bottom or lower aceticacid stream 174; the overhead stream 171 is rich in carbon monoxide, andthe bottom or lower acetic acid stream 174 is rich in methyl iodide,methyl acetate, and water. A portion 172 of the overhead stream (or gasstream) 171 is fed to the flasher (catalyst separation column) (2), andthe residual portion 173 of the overhead stream 171 is fed to a boilerand used as a heat source for the process or is atmosphericallydischarged from a flare stack or a vent stack. The residual portion 173of the overhead stream 171 may be incinerated or collected. The bottomor lower acetic acid stream 174 is fed to the diffusion column (18).

For example, the overhead stream 171 may have the following composition.

TABLE 52 Average molecular weight 26.22 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% 500 ppb to500 ppm 10%), e.g., 10 ppb to 3.6% (e.g., 100 ppb to 0.1%), or (e.g., 1to 100 ppm), or (e.g., 20 ppb to 2%), or less 20 ppb to 0.3% 50 ppb to0.1% than 7% (e.g., 1 ppt to 5%), (e.g., 100 ppb to 200 e.g., less than3.6% (e.g., 0.1 ppm) ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 5% (e.g., 0.001 to 2.5%) 0.01 to 2% 0.1 to1% CO 1 to 99% 5to 90% 10 to 85% CO₂ 0 to 5% (e.g., 0.01 to 5%) 0.1 to 3% 0.2 to 2% CH₄0.01 to 15% 0.1 to 10% 1 to 6% N₂ 0.01 to 20% 0.1 to 15% 1 to 10% AD 0to 1% (e.g., 0.001 to1%) 0.01 to 0.5% 0.02 to 0.2% MeOH 0 to 1% (e.g., 1ppm to 1%) 5 ppm to 0.5% 10 ppm to 0.1% MeI 0 to 90% (e.g., 1 to 90%) 5to 80% (e.g., 10 to 70%) 20 to 50% MA 0 to 5% (e.g., 0.001 to 2%) 0.01to 1% 0.05 to 0.5% H₂O 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppmto 0.1% AcOH 0 to 10% (e.g., 0.001 to 10%) 0.01 to 5% 0.1 to 2% HI 0 to1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to 0.1% FrOH 0 to 1% (e.g.,0.1 ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% PrOH 0 to 1% (e.g., 0.1ppm to 0.5%) 1 ppm to 0.2% 10 ppm to 0.1% DME 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 1 ppt to 0.1%)10 ppt to 100 ppm 100 ppt to 10 ppm EtOH 0 to 1% (e.g., 1 ppt to 0.1%)10 ppt to 100 ppm 100 ppt to 10 ppm EA 0 to 1% (e.g., 1 ppt to 0.1%) 10ppt to 100 ppm 100 ppt to 10 ppm EtI 0 to 1% (e.g., 1 ppt to 0.1%) 10ppt to 100 ppm 100 ppt to 10 ppm Li 0 to 0.1% (e.g., 1 ppt to 0.1%) 10ppt to 100 ppm 100 ppt to 10 ppm Rh 0 to 0.1% (e.g., 1 ppt to 0.1%) 10ppt to 100 ppm 100 ppt to 10 ppm

For example, the bottom acetic acid stream 174 may have the followingcomposition.

TABLE 53 Average molecular weight 59.64 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 10 pptto 300 ppm, ppb to 10%), e.g., 0.2 ppt to 1000 ppm), e.g., 100 ppt to100 ppb to 3.6% (e.g., 1 e.g., less than 700 ppm ppb to 2%) ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to0.1% CO 0 to 5% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.5% CO₂ 0to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% CH₄ 0 to 1%(e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% N₂ 0 to 1% (e.g., 1ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% AD 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.5% 10 ppm to 0.1% MeOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppmto 0.5% 50 ppm to 0.1% MeI 10 ppm to 10% 100 ppm to 5% 0.1 to 2% MA 10ppm to 10% 100 ppm to 5% 0.1 to 2% H₂O 10 ppm to 10% 100 ppm to 5% 0.1to 2% AcOH 80 to 99.9% 90 to 99.5% 97 to 99% HI 0 to 1% (e.g., 0.1 ppbto 0.5%) 1 ppb to 0.1% 10 ppb to 100 ppm FrOH 0 to 1% (e.g., 1 ppm to0.1%) 5 to 200 ppm 10 to 100 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1to 300 ppm 10 to 100 ppm DME 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100ppm 100 ppb to 20 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to100 ppm 100 ppb to 50 ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to100 ppm 100 ppb to 50 ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100ppm 100 ppb to 50 ppm EtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100ppm 100 ppb to 50 ppm Li 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to100 ppb 10 ppt to 10 ppb Rh 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 pptto 100 ppb 10 ppt to 10 ppb

(17) Low-Pressure Absorption Step

The noncondensable gas (the non-liquified gas in the decanter S2) 37,which is not condensed in the condenser of the first distillation step(3), and the noncondensable gas (the off-gas rich in acetic acid, methyliodide, and methyl acetate) 30 from the flasher (catalyst separationcolumn) (2) are mixed together to give a mixture (or mixed gas) 176. Themixture 176 is put into contact and scrub with an acetic acid 196 as anabsorption solvent in the low-pressure absorption column (17) of thelow-pressure absorption step or second absorption step (17), thusforming an overhead stream 181 and a bottom acetic acid stream 182; theoverhead stream 181 is rich in carbon monoxide, carbon dioxide, andnitrogen, and the bottom acetic acid stream 182 is rich in acetic acid,methyl iodide, and methyl acetate. The overhead stream 181 is mixed withthe overhead stream 171 from the high-pressure absorption column (16),and the resulting mixed gas 173 is fed to a boiler and is used as a heatsource for the process. A portion 184 of the bottom acetic acid stream182 is mixed with a portion of the bottom or lower acetic acid stream174 from the high-pressure absorption column (16), and the resultingmixtures is fed to the flasher (catalyst separation column) (2). Theresidual portion 183 of the bottom or lower acetic acid stream 182 ismixed with the bottom acetic acid stream 175 from the high-pressureabsorption column (16), and the resulting mixed acetic acid stream 185is fed to the diffusion column (18).

For example, the mixture (or mixed gas) 176 may have the followingcomposition.

TABLE 54 Average molecular weight 41.94 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 10%), 30 ppb to 1%(e.g., 500 ppb to 500 ppm e.g., 10 ppb to 3.6% (e.g., 20 ppb to 100 ppbto 0.1%), or (e.g., 1 to 100 ppm), or 2%), or less than 7% (e.g., 1 pptto 20 ppb to 0.3% 50 ppb to 0.1% 5%), e.g., less than 3.6% (e.g., 0.1(e.g., 100 ppb to ppb to 2%), e.g., 1 ppb to 1% (e.g., 200 ppm) 10 ppbto 0.5%) H₂ 0.01 to 5% 0.05 to 2% 0.1 to 1% CO 1 to 99% 5 to 80% 10 to70% CO₂ 0.1 to 20% 0.5 to 15% 1 to 15% CH₄ 0.1 to 20% 0.5 to 15% 1 to10% N₂ 0.1 to 20% 0.5 to 15% 1 to 10% AD 0 to 10% (e.g., 0.001 to 7%)0.01 to 5% 0.1 to 3% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10ppm to 0.1% MeI 1 to 95% 10 to 90% 20 to 80% MA 0.01 to 40% 0.1 to 20% 1to 10% H₂O 0 to 20% (e.g., 0.01 to 20%) 0.02 to 10% 0.05 to 1% AcOH 0 to10% (e.g., 0.001 to 10%) 0.01 to 1% 0.03 to 0.5% HI 0 to 1% (e.g., 1 pptto 0.1%) 100 ppt to 0.01% 10 ppb to 1 ppm FrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 100 ppm 100 ppb to 10 ppm PrOH 0 to 1% (e.g., 1 ppb to0.1%) 10 ppb to 100 ppm 100 ppb to 10 ppm DME 0 to 1% (e.g., 0.1 ppm to1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 0.01 ppb to100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm EtOH 0 to 1% (e.g., 0.01 ppbto 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm EA 0 to 1% (e.g., 0.01 ppbto 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm EtI 0 to 1% (e.g., 0.01 ppbto 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm Li 0 to 0.1% (e.g., 0.01ppb to 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm Rh 0 to 0.1% (e.g.,0.01 ppb to 100 ppm) 0.1 ppb to 10 ppm 1 ppb to 1 ppm

For example, the overhead stream (the line 181) may have the followingcomposition.

TABLE 55 Average molecular weight 26.57 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% (e.g., 500ppb to 500 ppm 10%), e.g., 10 ppb to 3.6% (e.g., 100 ppb to 0.1%), or(e.g., 1 to 100 ppm), or 20 ppb to 2%), or less than 7% 20 ppb to 0.3%50 ppb to 0.1% (e.g., (e.g., 1 ppt to 5%), e.g., less 100 ppb to 200ppm) than 3.6% (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0.01 to 10% 0.1 to 5% 0.2 to 2% CO 10 to 90% 20 to 80% 40 to75% CO₂ 0.1 to 40% 1 to 30% 5 to 20% CH₄ 0.1 to 20% 0.5 to 15% 1 to 10%N₂ 0.1 to 20% 1 to 15% 2 to 10% AD 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppmto 0.5% 10 ppm to 0.1% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10ppm to 0.1% MeI 0 to 1% (e.g., 0.01 ppm to 1%) 0.1 ppm to 0.1% 1 to 100ppm MA 0 to 5% (e.g., 0.001 to 5%) 0.01 to 1% 0.05 to 0.5% H₂O 0 to 1%(e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to 0.1% AcOH 0 to 30% (e.g.,0.001 to 30%) 0.01 to 10% 0.1 to 5% HI 0 to 1% (e.g., 1 ppb to 1%) 10ppb to 0.1% 100 ppb to 0.01% FrOH 0 to 1% (e.g., 0.01 ppm to 0.5%) 0.1ppm to 0.1% 10 ppm to 0.01% PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% DME 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm100 ppt to 10 ppm EtOH 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm100 ppt to 10 ppm EA 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100ppt to 10 ppm EtI 0 to 1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100ppt to 10 ppm Li 0 to 0.1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100ppt to 10 ppm Rh 0 to 0.1% (e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100ppt to 10 ppm

For example, the bottom acetic acid stream (the line 182) may have thefollowing composition.

TABLE 56 Average molecular weight 63.17 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 10 pptto 300 ppm, ppb to 10%), e.g., 0.2 ppt to 1000 ppm), e.g., 100 ppt to100 ppb to 3.6% (e.g., 1 e.g., less than 700 ppm ppb to 2%) ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to0.1% CO 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.5% CO₂ 0to 5% (e.g., 1 ppm to 3%) 10 ppm to 1% 50 ppm to 0.5% CH₄ 0 to 1% (e.g.,1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% N₂ 0 to 1% (e.g., 1 ppm to1%) 10 ppm to 0.5% 50 ppm to 0.1% AD 0 to 5% (e.g., 1 ppm to 2%) 10 ppmto 1% 100 ppm to 0.5% MeOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50ppm to 0.1% MeI 100 ppm to 30% 0.1 to 20% 1 to 15% MA 10 ppm to 10% 100ppm to 5% 0.1 to 2% H₂O 10 ppm to 10% 100 ppm to 5% 0.1 to1% AcOH 70 to99% 80 to 98% 85 to 95% HI 0 to 1% (e.g., 1 ppb to 0.5%) 10 ppb to 0.1%100 ppb to 100 ppm FrOH 0 to 1% (e.g., 1 ppm to 0.1%) 5 to 100 ppm 10 to50 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 300 ppm 10 to 100 ppmDME 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 20 ppm(CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppmEtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm Li0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100 ppb 10 ppt to 10 ppbRh 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100 ppb 10 ppt to 10ppb

(18) Diffusion Step

In the diffusion column (stripping column) of the diffusion step (18),the mixed acetic acid stream 185 is distilled and stripped to form anoverhead stream 191 and a bottom acetic acid stream 194; the overheadstream 191 is rich in methyl iodide and acetic acid (and also containsmethyl acetate, acetaldehyde, or other compounds), and the bottom aceticacid stream 194 is rich in acetic acid, methyl acetate, and water. Afirst portion of the bottom acetic acid stream 194 is heated by aheating unit and is returned to a lower part of the diffusion column(18). A second portion (or residual portion) of the bottom acetic acidstream 194 is mixed with a portion 65 of the condensate (a portion ofthe condensate that is rich in acetic acid and is held in the hold tankT3) of the third overhead 61 from the third distillation column (6),forming a mixture 195. A portion 197 of the resulting mixture 195 isrecycled to an upper part of the high-pressure absorption column (16),and the residual portion 196 of the mixture 195 is recycled to an upperpart of the low-pressure absorption column (17).

The overhead stream 191 is cooled and condensed in a condenser to form anoncondensable gas 192 and a condensate 193. The noncondensable gas 192(a stream being rich in methyl iodide and carbon monoxide and alsocontaining carbon dioxide, methane, ethyl acetate, acetaldehyde, orother compounds) is mixed with the noncondensable gas from the decanterS2 of the liquid-liquid separation step (10), and the mixture is cooledand condensed in a condenser. The condensate (a stream being rich inmethyl iodide, acetic acid, and methyl acetate and also containingwater, acetaldehyde, or other compounds) 193 of the overhead stream 191is fed to the hold tank T1 for holding the condensates 26, 28 of thevolatile phase 24 from the flasher (catalyst separation column) (2). Thecondensate 193 is recycled to the reactor (1) via the hold tank T1. Thecondensate 193 may directly be recycled to the reactor (1).

For example, the mixed acetic acid stream 185 may have the followingcomposition.

TABLE 57 Average molecular weight 62.31 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 less than 1% (e.g., 1 10 pptto 300 ppm, ppb to 10%), e.g., 0.2 ppt to 1000 ppm), e.g., 100 ppt to100 ppb to 3.6% (e.g., 1 e.g., less than 700 ppm ppb to 2%) ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to0.1% CO 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.5% CO₂ 0to 2% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.2% CH₄ 0 to 1%(e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% N₂ 0 to 1% (e.g., 1ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% AD 0 to 2% (e.g., 1 ppm to 2%)10 ppm to 1% 100 ppm to 0.5% MeOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to0.5% 50 ppm to 0.1% MeI 100 ppm to 30% 0.1% to 20% 1 to 15% MA 10 ppm to10% 100 ppm to 5% 0.1 to 2% H₂O 10 ppm to 10% 100 ppm to 5% 0.1 to 1%AcOH 70 to 99% 80 to 98% 85 to 95% HI 0 to 1% (e.g., 1 ppb to 0.5%) 10ppb to 0.1% 100 ppb to 100 ppm FrOH 0 to 1% (e.g., 1 ppm to 0.1%) 5 to100 ppm 10 to 50 ppm PrOH 0 to 1% (e.g., 0.1 ppm to 0.1%) 1 to 300 ppm10 to 100 ppm DME 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100ppb to 50 ppm (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm100 ppb to 50 ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm100 ppb to 50 ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100ppb to 50 ppm EtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100ppb to 50 ppm Li 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100 ppb10 ppt to 10 ppb Rh 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100ppb 10 ppt to 10 ppb

For example, the overhead stream 191 may have the following composition.

TABLE 58 Average molecular weight 95.18 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% (e.g., 500ppb to 500 ppm 10%), e.g., 10 ppb to 3.6% (e.g., 100 ppb to 0.1%), or(e.g., 1 to 100 ppm), or 20 ppb to 2%), or less than 7% 20 ppb to 0.3%50 ppb to 0.1% (e.g., (e.g., 1 ppt to 5%), e.g., less 100 ppb to 200ppm) than 3.6% (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 2% (e.g., 0.1 ppm to 2%) 1 ppm to 1% 10 ppm to 0.1% CO0.001 to 10% 0.01 to 5% 0.1 to 2% CO₂ 0.001 to 10% 0.01 to 5% 0.1 to 2%CH₄ 0 to 5% (e.g., 1 ppm to 5%) 10 ppm to 2% 100 ppm to 1% N₂ 0 to 2%(e.g., 0.1 ppm to 2%) 1 ppm to 1% 10 ppm to 0.1% AD 10 ppm to 5% 100 ppmto 2% 0.1 to 1% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to 0.5% 10 ppm to0.1% MeI 5 to 99% 10 to 90% 30 to 80% MA 0.01 to 30% 0.1 to 20% 1 to 10%H₂O 0.001 to 10% 0.01 to 5% 0.1 to 2% AcOH 0.1 to 50% 1 to 40% 5 to 30%HI 0 to 1% (e.g., 1 ppb to 1%) 10 ppb to 0.1% 100 ppb to 0.01% FrOH 0 to1% (e.g., 0.01 ppm to 0.5%) 0.1 ppm to 0.1% 1 ppm to 0.01% PrOH 0 to 1%(e.g., 0.01 ppm to 0.5%) 0.1 ppm to 0.1% 1 ppm to 0.01% DME 0 to 1%(e.g., 0.1 ppm to 1%) 1 ppm to 0.1% 5 ppm to 0.05% (CH₃)₂C═O 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm EtOH 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm EA 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm EtI 0 to 1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm Li 0 to 0.1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm Rh 0 to 0.1%(e.g., 1 ppt to 0.1%) 10 ppt to 100 ppm 100 ppt to 10 ppm

For example, the bottom acetic acid stream 194 may have the followingcomposition.

TABLE 59 Average molecular weight 59.59 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb less than 1% (e.g., 1 10ppt to 300 ppm, to 10%), e.g., 0.2 ppb to ppt to 1000 ppm), e.g., 100ppt to 100 3.6% (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppbto 0.001% CO 0 to 2% (e.g., 1 ppb to 1%) 10 ppb to 0.5% 100 ppb to 0.3%CO₂ 0 to 2% (e.g., 1 ppb to 1%) 10 ppb to 0.5% 100 ppb to 0.3% CH₄ 0 to1% (e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% N₂ 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% AD 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% MeOH 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% MeI 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 0.01% 100 ppb to 0.001% MA 0 to 2%(e.g., 1 ppb to 1%) 10 ppb to 0.5% 100 ppb to 0.1% H₂O 0 to 5% (e.g., 10ppm to 5%) 100 ppm to 2% 0.1 to 1% AcOH 80 to 100% 90 to 99.99% 98 to99.9% HI 0 to 1% (e.g., 0.1 ppb to 0.5%) 1 ppb to 0.1% 10 ppb to 100 ppmFrOH 0 to 1% (e.g., 1 to 100 ppm) 5 to 1000 ppm 10 to 300 ppm PrOH 0 to1% (e.g., 0.1 ppm to 0.1%) 1 to 1000 ppm 10 to 200 ppm DME 0 to 1%(e.g., 1 ppb to 100 ppm) 10 ppb to 10 ppm 100 ppb to 5 ppm (CH₃)₂C═O 0to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm EtOH 0to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm EA 0 to1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm EtI 0 to 1%(e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm Li 0 to 0.1%(e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm Rh 0 to 0.1%(e.g., 0.01 ppt to 100 ppb) 1 ppt to 10 ppb 10 ppt to 1 ppb

For example, the composition of the portions 197, 196 of the mixture 195may be substantially the same as the composition of the bottom aceticacid stream 194.

For example, the noncondensable gas 192 of the overhead stream 191 mayhave the following composition.

TABLE 60 Average molecular weight 73.51 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 10 ppb to 30 ppb to 1% (e.g., 500ppb to 500 ppm 10%), e.g., 10 ppb to 3.6% (e.g., 100 ppb to 0.1%), or(e.g., 1 to 100 ppm), or 20 ppb to 2%), or less than 7% 20 ppb to 0.3%50 ppb to 0.1% (e.g., (e.g., 1 ppt to 5%), e.g., less 100 ppb to 200ppm) than 3.6% (e.g., 0.1 ppb to 2%), e.g., 1 ppb to 1% (e.g., 10 ppb to0.5%) H₂ 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.2% CO 0.1to 90% 1 to 60% 5 to 30% CO₂ 0.1 to 90% 1 to 60% 5 to 30% CH₄ 0.01 to20% 0.1 to 10% 0.5 to 5% N₂ 0.01 to 20% 0.1 to 10% 0.3 to 3% AD 100 ppmto 20% 0.1 to 10% 0.5 to 5% MeOH 0 to 1% (e.g., 1 ppm to 1%) 5 ppm to0.5% 10 ppm to 0.1% MeI 1 to 95% 10 to 90% 40 to 80% MA 0.01 to 20% 0.1to 10% 0.5 to 5% H₂O 10 ppm to 5% 100 ppm to 2% 500 ppm to 1% AcOH 10ppm to 10% 100 ppm to 5% 0.1 to 2% HI 0 to 1% (e.g., 1 ppb to 1%) 10 ppbto 0.1% 100 ppb to 0.01% FrOH 0 to 1% (e.g., 0.01 ppm to 0.5%) 0.1 ppmto 0.1% 1 ppm to 0.01% PrOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to0.01% 100 ppb to 0.001% DME 0 to 1% (e.g., 0.1 ppm to 1%) 1 ppm to 0.1%5 ppm to 0.05% (CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm100 ppt to 10 ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm100 ppt to 10 ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100ppt to 10 ppm EtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100ppt to 10 ppm Li 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm 100ppt to 0.01 ppm Rh 0 to 0.1% (e.g., 1 ppt to 1 ppm) 10 ppt to 0.1 ppm100 ppt to 0.01 ppm

For example, the condensate 193 of the overhead stream 191 may have thefollowing composition.

TABLE 61 Average molecular weiqht 97.27 Range Preferred range Morepreferred range O₂ 10% or less (e.g., 0.1 ppb to less than 1% (e.g., 110 ppt to 300 ppm, 10%), e.g., 0.2 ppb to 3.6% ppt to 1000 ppm), e.g.,100 ppt to 100 (e.g., 1 ppb to 2%) e.g., less than 700 ppm ppm (e.g., 1ppt to 500 ppm) H₂ 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50 ppm to0.1% CO 0 to 2% (e.g., 1 ppm to 2%) 10 ppm to 1% 100 ppm to 0.5% CO₂ 0to 3% (e.g., 1 ppm to 1%) 10 ppm to 1% 50 ppm to 0.5% CH₄ 0 to 1% (e.g.,1 ppm to 1%) 10 ppm to 0.5% 50 ppm to 0.1% N₂ 0 to 1% (e.g., 1 ppm to1%) 10 ppm to 0.5% 50 ppm to 0.1% AD 0 to 3% (e.g., 1 ppm to 2%) 10 ppmto 1.5% 100 ppm to 1% MeOH 0 to 1% (e.g., 1 ppm to 1%) 10 ppm to 0.5% 50ppm to 0.1% MeI 10 to 95% 30 to 90% 50 to 80% MA 0.1 to 30% 1 to 20% 3to 10% H₂O 0.001 to 10% 0.01 to 5% 0.1 to 2% AcOH 1 to 70% 5 to 50% 10to 35% HI 0 to 1% (e.g., 1 ppb to 0.5%) 10 ppb to 0.1% 100 ppb to 100ppm FrOH 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 100 ppm 1 to 50 ppmPrOH 0 to 1% (e.g., 0.01 ppm to 0.1%) 0.1 to 300 ppm 1 to 100 ppm DME 0to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 20 ppm(CH₃)₂C═O 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm EtOH 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50ppm EA 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppmEtI 0 to 1% (e.g., 1 ppb to 0.1%) 10 ppb to 100 ppm 100 ppb to 50 ppm Li0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100 ppb 10 ppt to 10 ppbRh 0 to 0.1% (e.g., 0.1 ppt to 1000 ppb) 1 ppt to 100 ppb 10 ppt to 10ppb

The off-gas treatment section (15) usually comprises at least oneabsorption step selected from the high-pressure absorption step (16) andthe low-pressure absorption step (17) among these steps (16) to (18).

EXAMPLES

The following examples are intended to describe this invention infurther detail and should by no means be interpreted as defining thescope of the invention.

For corrosion tests, the following test pieces were used.

[Test piece]

Zr: zirconium, manufactured by ATI Japan

HB2: HASTELLOY B2 (nickel-based alloy), manufactured by Oda Koki Co.,Ltd.

HC276: HASTELLOY C (nickel-based alloy), manufactured by Oda Koki Co.,Ltd.

SUS316: Stainless steel, manufactured by Oda Koki Co., Ltd.

The following test items for corrosion were evaluated.

[Corrosion Rate of Test Piece]

The weight of each test piece after corrosion test was measured todetermine a corrosion rate. Specifically, the decrease in weight of thetest piece by corrosion was measured to determine a corrosion rate ofthe test piece per year, and the corrosion rate was converted into adecrease in thickness (mm) of the test piece per year (unit: “mm/Y”),thus evaluating the corrosion amount (corrosion rate).

[Presence of Partial Corrosion]

The presence of partial corrosion of each test piece was visuallyobserved. Incidentally, the “partial corrosion” includes bead corrosion,pitting corrosion, and spot corrosion.

[Degree of Coloration]

The APHA (Hazen color number) of a mixture was measured according toJapanese Industrial Standards (JIS). A larger APHA value means a largerdegree of coloration.

[Compositions (Component Ratios) of Liquid Phase and Gaseous Phase]

In the compositions (component ratios) shown in Comparative Examples andExamples and Tables, the concentrations of organic matters and waterwere measured by gas chromatography, and the concentration of lithiumiodide (LiI) was measured by atomic absorption analysis. Theconcentration of hydrogen iodide (HI) was calculated by subtracting theconcentration of an iodide ion derived from an iodide ion from the totaliodide ion (I⁻) concentration. The total amount of components in each ofthe liquid and the gaseous phases, including impurities and minor (ortrace) components, is 100%. In this respect, the total of the componentsshown in Tables may be inconsistent with 100% in some cases due toanalysis error and rounding up or down to significant figures (orpredetermined digit number).

Incidentally, for each of noncondensable gaseous components (hydrogen,carbon monoxide, carbon dioxide, methane, nitrogen, oxygen) other thanmethyl iodide, when a component had a concentration of less than 1% byweight and less than 1% by volume, the concentration of the componentwas rounded off to two significant digits; when a component had aconcentration of 1% by weight or more and 1% by volume or more, theconcentration of the component was rounded off to the closest wholenumber.

Comparative Examples 1 to 14 and Examples 1 to 22 Comparative Example 1

In an autoclave (capacity: 500 ml) made of zirconium Zr, 39 g of MeI,6.3 g of water, 0.003 g of HI, 8.4 g of MA, 207 g of acetic acid, 0.03 gof PA, and 46 g of LiI were charged, and a test piece (size: 36mm×25=mm×2.5 mm) was set in the autoclave, and the autoclave was closedwith a lid. Oxygen dissolved in the liquid in the autoclave was replacedby bubbling nitrogen gas N₂, and then 0.003 g of DME and 0.06 g of ADwere fed to the autoclave. Increasing the pressure of the autoclave fromthe atmospheric pressure to 1 MPa with nitrogen gas N₂ and thenreleasing the pressure to the atmospheric pressure were performed threetimes; and increasing the pressure of the autoclave to 1 MPa with amixed gas (93 vol % of CO, 7 vol % of O₂) and then releasing thepressure to the atmospheric pressure were performed three times; and amixed gas (93 vol % of CO, 7 vol % of O₂) was injected until thepressure of the autoclave was 4 MPa. After injection of the mixed gas,the pressure was gradually released, and the oxygen concentration in thereleased gas was measured by an oxygen analyzer [a galvanic oxygenanalyzer (“Model 1000RS” manufactured by TEKHNE Corporation)] todetermine that the oxygen concentration reached 7 vol %. Thereafter, thepressure of the autoclave was released to 1 MPa, and then the autoclavewas heated to 190° C. in an oil bath. The pressure after the heating wasmaintained to 2.8 MPa. After 100 hours under the steady condition, theautoclave was cooled to a room temperature, and the reaction mixture wassampled from a nozzle of the autoclave. The sample was subjected tocomposition analysis to determine a degree of coloration (APHA).Moreover, the atmosphere in the autoclave was replaced with N₂, and theautoclave was opened. The test piece was taken out and weighed todetermine a corrosion rate.

The concentration of oxygen in the gaseous phase was 7 vol %. Theconcentration of oxygen dissolved in the liquid phase under a totalpressure of 1 MPa was calculated to be 7.0×10⁻⁵ g/g by Aspen Plus(manufactured by Aspen Technology, Inc.).

Comparative Examples 2 to 11

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the pressure, and theheating temperature were changed.

Comparative Example 12

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (93vol % of N₂, 7 vol % of O₂), the pressure, and the heating temperaturewere changed.

Comparative Example 13

In the process for continuously producing acetic acid as shown in FIG.1, methanol was allowed to react with carbon monoxide (carbon monoxidehaving a concentration of oxygen of 10% by weight (9 vol %)) in acarbonylation reactor, the reaction mixture was continuously fed, forflash evaporation, from the reactor to a flasher to form a less-volatilephase (a bottom fraction at least containing a rhodium catalyst, lithiumiodide, acetic acid, methyl acetate, methyl iodide, water, and hydrogeniodide) and a volatile phase (liquid temperature of liquefied gaseousfraction: 140° C.), and the volatile phase was fed to a firstdistillation column.

The volatile phase contained 26.8% by weight of methyl iodide (MeI),4.5% by weight of methyl acetate (MA), 2.0% by weight of water (H₂O),500 ppm by weight of hydrogen iodide (HI), 600 ppm by weight ofacetaldehyde (AD), 62.8% by weight of acetic acid, 0.0070% by weight (70ppm by weight) of hydrogen, 2% by weight of carbon monoxide, 0.060% byweight (600 ppm by weight) of carbon dioxide, 0.070% by weight (700 ppmby weight) of methane, 0.070% by weight (700 ppm by weight) of nitrogen,and oxygen and other minor components (total: 100% by weight).

In a case where acetic acid vapor and/or liquid enters a pressure sensorof a differential pressure type level gauge for measuring a liquid levelof the bottom liquid of the first distillation column, the level gaugemay be operated improperly. In order to prevent such a trouble, 9 partsby weight of purge air relative to 100 parts by weight of the volatilephase fed to the first distillation column was supplied to the gaseousphase side of the differential pressure type level gauge.

The volatile phase (100 parts by weight) was fed to the firstdistillation column (actual number of plates: 20 plates, feed plate: the2nd plate from the bottom) and was distilled under conditions of at agauge pressure of 150 kPa, a column bottom temperature of 143° C., acolumn top temperature of 115° C., and a light-phase reflux ratio of 12.The resulting overhead was cooled in a condenser to form a condensateand a noncondensable gas. The condensate (temperature: 40° C.) wasliquid-liquid (or biphasically) separated in a decanter to form anaqueous phase (light phase) and an organic phase (heavy phase), and 1.3parts by weight of the aqueous phase and 30 parts by weight of theorganic phase were recycled to the reactor. From the condenser, 13 partsby weight of the noncondensable gas (off-gas stream) was withdrawn.

The overhead (column top) composition from the first distillation column(the composition of the overhead) was as follows: 43.2% by weight ofmethyl iodide (MeI), 7.5% by weight of methyl acetate (MA), 21.1% byweight of water (H₂O), 100 ppm by weight of hydrogen iodide (HI), 5.9%by weight of acetic acid, 0.010% by weight (100 ppm by weight) ofhydrogen, 4% by weight of carbon monoxide, 0.10% by weight (1000 ppm byweight) of carbon dioxide, 0.11% by weight (1100 ppm by weight) ofmethane, 12% by weight of nitrogen, 6% by weight (7 vol %) of oxygen,and other minor components (total: 100% by weight); and the compositionof the noncondensable gas (off-gas stream) from the condenser was asfollows: 3.6% by weight of methyl iodide (MeI), 0.2% by weight of methylacetate (MA), 200 ppm by weight of water (H₂O), hydrogen iodide (HI)(not measured), 200 ppm by weight of acetic acid, 0.040% by weight (400ppm by weight) of hydrogen, 17% by weight of carbon monoxide, 0.50% byweight of carbon dioxide, 0.50% by weight of methane, 53% by weight ofnitrogen, 25% by weight (23 vol %) of oxygen, and other minor components(total: 100% by weight). The composition of the aqueous phase (lightphase) from the condenser was as follows: 3.3% by weight of methyliodide (MeI), 6.6% by weight of methyl acetate (MA), 73.0% by weight ofwater (H₂O), 100 ppm by weight of hydrogen iodide (HI), 17.0% by weightof acetic acid, 0.0080% by weight (80 ppm by weight) of oxygen, andother minor components (total: 100% by weight); and the composition ofthe organic phase (heavy phase) was as follows: 86% by weight of methyliodide (MeI), 11.1% by weight of methyl acetate (MA), 0.5% by weight ofwater (H₂O), 100 ppm by weight of hydrogen iodide (HI), 2.0% by weightof acetic acid, 0.0090% by weight (90 ppm by weight) of oxygen, andother minor components (total: 100% by weight).

A side-cut stream (62.8 parts by weight) of the first distillationcolumn was fed to a second distillation column for dehydration andpurification. The composition of the above side-cut stream was asfollows: 2.4% by weight of methyl iodide (MeI), 1.6% by weight of methylacetate (MA), 1.3% by weight of water (H₂O), 45 ppm by weight ofhydrogen iodide (HI), 94.6% by weight of acetic acid, 0.0090% by weight(90 ppm by weight) of oxygen, and other minor components (total: 100% byweight). The remainder of the feed (volatile phase) was recycled as abottom stream to the reaction system. The term “parts by weight” of afluid (e.g., a volatile phase, an aqueous phase (light phase) and anorganic phase (heavy phase), an off-gas stream, a side-cut stream, and abottom stream) indicates a flow rate per unit hour (per hour) (the sameapplies hereinafter).

In such a continuous reaction process, the above-mentioned test pieceswere placed on the feed plate of the first distillation column (the 2ndplate from the bottom, temperature: 140° C.) and the upper part of thecolumn (the 19th plate from the bottom). After the process was operatedfor 500 hours, each test piece was examined for a corrosion test. Theweight of each test piece before and after the corrosion test wasmeasured to determine a corrosion amount.

Moreover, the crude acetic acid (side-cut stream) from the firstdistillation column was examined for the APHA.

Comparative Example 14

In the process for continuously producing acetic acid as shown in FIG.1, methanol was allowed to react with carbon monoxide (concentration ofoxygen: 10 ppm by weight) in a carbonylation reactor. In the same manneras in Comparative Example 13, a volatile phase from a flasher wasdistilled in a first distillation column, and 100 parts by weight of aside-cut stream of the first distillation column was fed to a seconddistillation column for dehydration and purification. In a case whereacetic acid vapor and/or liquid enters a pressure sensor of adifferential pressure type level gauge for measuring a liquid level ofthe bottom liquid of the second distillation column, the level gauge maybe operated improperly. In order to prevent such a trouble, 11 parts byweight of purge air relative to 100 parts by weight of the side-cutstream fed was supplied to the gaseous phase side of the differentialpressure type level gauge.

In the second distillation column (actual number of plates: 50 plates,distance or place spacing between a feed plate and a plate from which acolumn-top vapor is withdrawn: 15 plates as actual plates), distillationwas carried out at a column top gauge pressure of 200 kPa, a columnbottom temperature of 161° C., a column top temperature of 150° C., anda reflux ratio of 0.5 (reflux amount/distillate amount).

From the top of the second distillation column, 60 parts by weight of anoverhead was withdrawn. The overhead had a composition containing 6.3%by weight of methyl iodide (MeI), 4.1% by weight of methyl acetate (MA),3.3% by weight of water (H₂O), 10 ppm by weight of hydrogen iodide (HI),0 ppm by weight of hydrogen, 0.00010% by weight (1.0 ppm by weight) ofcarbon monoxide, 0 ppm by weight of carbon dioxide, 0 ppm by weight ofmethane, 14% by weight of nitrogen, 4% by weight (7 vol %) of oxygen,and other minor components, and the remainder was acetic acid.

The overhead from the second distillation column was cooled in acondenser, and the resulting condensate was held in a reflux tank. Aportion (32 parts by weight) of the condensate in the tank was withdrawnand was recycled to the reaction system. A portion (16 parts by weight)of the condensate was returned to the second distillation column forreflux at a reflux ratio of 0.5 as mentioned above. The condensate had acomposition containing 7.7% by weight of methyl iodide (MeI), 5.0% byweight of methyl acetate (MA), 4.1% by weight of water (H₂O), 9 ppm byweight of hydrogen iodide (HI), 0.0070% by weight (70 ppm by weight) ofoxygen, and other minor components, and the remainder was acetic acid.From the condenser, 11 parts by weight of a noncondensable gas waswithdrawn. The noncondensable gas had a composition having 22% by weight(20 vol %) of oxygen and 78% by weight (70 vol %) of nitrogen, andnegligible amounts of other components.

A crude acetic acid obtained by the dehydration and purification waswithdrawn as a bottom stream from the second distillation column. Thebottom stream (crude acetic acid) has a composition containing 6 ppb byweight of methyl iodide (MeI), 0.05% by weight of water (H₂O), 4 ppb byweight of hydrogen iodide (HI), 6 ppm by weight of methyl acetate (MA),and other minor components (containing oxygen), and the remainder wasacetic acid. The term “parts by weight” of a fluid (e.g., a feed liquid,an overhead (distillate), an off-gas stream, and a bottom stream)indicates a flow rate per unit hour (per hour) (the same applieshereinafter).

In such a continuous reaction process, the above-mentioned test pieceswere placed on the 2nd plate from the bottom (a plate above an air purgeline) of the second distillation column and the column top (the 50thplate from the bottom) of the second distillation column and were leftfor 500 hours, and each test piece was examined for a corrosion test.The weight of each test piece before and after the corrosion test wasmeasured to determine a corrosion amount.

Moreover, the feed liquid to the second distillation column (theside-cut stream from the first distillation column) and the bottomstream (crude acetic acid) from the second distillation column wereexamined for the APHA.

Examples 1 to 10

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.010 vol % andcarbon monoxide CO as the remainder), the pressure, and the heatingtemperature were changed.

Example 11

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (95vol % of CO, 5 vol % of O₂), the pressure, and the heating temperaturewere changed. The concentration of oxygen in the gaseous phase was 1 vol%. The concentration of oxygen dissolved in the liquid phase under atotal pressure of 140 kPa was calculated to be 4.0×10⁻⁶ g/g by AspenPlus (manufactured by Aspen Technology, Inc.).

Example 12

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (99vol % of CO, 1 vol % of O₂), the pressure, and the heating temperaturewere changed.

Example 13

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.00010 vol % (1.0vol ppm) and carbon monoxide CO as the remainder), the pressure, and theheating temperature were changed. Provided that, until the concentrationof oxygen in a released gas was 1.0 vol ppm, a pressurization to 1 MPaGwith a mixed gas (a mixed gas containing oxygen O₂ at a concentration of0.00010 vol % and carbon monoxide CO as the remainder) and a pressurerelease to the atmospheric pressure were repeated four times. That is,as a result, the pressurization to 1 MPaG and the pressure release tothe atmospheric pressure were repeated seven times in total, andthereafter, the pressure was increased to 4 MPa and was then released.The oxygen concentration in the released gas was measured by an oxygenanalyzer to determine that the oxygen concentration reached 1.0 vol ppm.Then, the corrosion test was performed in the same manner as inComparative Example 1.

Example 14

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.10 vol % andnitrogen N₂ as the remainder), the pressure, and the heating temperaturewere changed.

Example 15

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.010 vol % andnitrogen N₂ as the remainder), the pressure, and the heating temperaturewere changed.

Example 16

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.0010 vol % andnitrogen N₂ as the remainder), the pressure, and the heating temperaturewere changed. Provided that, until the concentration of oxygen in areleased gas was 0.0010 vol %, a pressurization to 1 MPaG with a mixedgas (a mixed gas containing oxygen O₂ at a concentration of 0.0010 vol %and N₂ as the remainder) and a pressure release to the atmosphericpressure were repeated three times. That is, as a result, the pressurerelease to the atmospheric pressure and the pressurization to 1 MPaGwere repeated six times in total, and thereafter, the pressure wasincreased to 4 MPa and was then released. The oxygen concentration inthe released gas was measured by an oxygen analyzer to determine thatthe oxygen concentration reached 0.0010 vol %. Then, the corrosion testwas performed in the same manner as in Comparative Example 1.

Example 17

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing 2 vol % of H₂, 15 vol % of CO₂, 7 vol % of CH₄, 8vol % of N₂, 0.010 vol % of O₂, and carbon monoxide CO as theremainder), the pressure, and the heating temperature were changed.

Example 18

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.00010 vol % (1.0vol ppm) and carbon monoxide CO as the remainder), the pressure, and theheating temperature were changed. Provided that, until the concentrationof oxygen in a released gas was 1.0 vol ppm, a pressurization to 1 MPaGwith a mixed gas (a mixed gas containing oxygen O₂ at a concentration of0.00010 vol % and carbon monoxide CO as the remainder) and a pressurerelease to the atmospheric pressure were repeated four times. That is,as a result, the pressure release to the atmospheric pressure and thepressurization to 1 MPaG were repeated seven times in total, andthereafter, the pressure was increased to 4 MPa and was then released.The oxygen concentration in the released gas was measured by an oxygenanalyzer to determine that the oxygen concentration reached 1.0 vol ppm.Then, the corrosion test was performed in the same manner as inComparative Example 1.

Example 19

A corrosion test was performed in the same manner as in ComparativeExample 1 except that the feed composition, the feed gas composition (amixed gas containing oxygen O₂ at a concentration of 0.000010 vol %(0.10 vol ppm) and carbon monoxide CO as the remainder), the pressure,and the heating temperature were changed. Provided that, until theconcentration of oxygen in a released gas was 0.10 vol ppm, apressurization to 1 MPaG with a mixed gas (a mixed gas containing oxygenO₂ at a concentration of 0.000010 vol % and carbon monoxide CO as theremainder) and a pressure release to the atmospheric pressure wererepeated four times. That is, as a result, the pressure release to theatmospheric pressure and the pressurization to 1 MPaG were repeatedseven times in total, and thereafter, the pressure was increased to 4MPa and was then released. The oxygen concentration in the released gaswas measured by an oxygen analyzer to determine that the oxygenconcentration reached 0.10 vol ppm. Then, the corrosion test wasperformed in the same manner as in Comparative Example 1.

Example 20

A corrosion test was performed in the same manner as in ComparativeExample 13 except that the concentration of oxygen in the volatile phasefrom the flasher was changed from 3% by weight to 0.1% by weight byregulating the concentration of oxygen in carbon monoxide fed to thecarbonylation reactor. By reducing the concentration of oxygen in carbonmonoxide, the composition of the volatile phase fed to the firstdistillation column was changed. The volatile phase contained 27.6% byweight of methyl iodide (MeI), 4.6% by weight of methyl acetate (MA),2.0% by weight of water (H₂O), 450 ppm by weight of hydrogen iodide(HI), 64.6% by weight of acetic acid, 0.0070% by weight (70 ppm byweight) of hydrogen, 0.60% by weight (6000 ppm by weight) of carbonmonoxide, 0.070% by weight (700 ppm by weight) of carbon dioxide, 0.070%by weight (700 ppm by weight) of methane, 0.070% by weight (700 ppm byweight) of nitrogen, 0.30% by weight (0.60 vol %) of oxygen, and otherminor components (total: 100% by weight). Moreover, the overhead (columntop) composition from the first distillation column (the composition ofthe overhead) was as follows: 54.2% by weight of methyl iodide (MeI),9.4% by weight of methyl acetate (MA), 26.5% by weight of water (H₂O),100 ppm by weight of hydrogen iodide (HI), 7.3% by weight of aceticacid, 0.010% by weight (100 ppm by weight) of hydrogen, 1% by weight ofcarbon monoxide, 0.14% by weight (1400 ppm by weight) of carbon dioxide,0.15% by weight (1500 ppm by weight) of methane, 0.15% by weight (1500ppm by weight) of nitrogen, 0.20% by weight (2000 ppm by weight) (0.30vol %) of oxygen, and other minor components (total: 100% by weight).From the condenser for cooling the column top overhead, 1.4 parts byweight of a noncondensable gas (off-gas stream) was withdrawn. Thecomposition of the noncondensable gas was as follows: 35% by weight ofmethyl iodide (MeI), 2.0% by weight of methyl acetate (MA), 1000 ppm byweight of water (H₂O), hydrogen iodide (HI) (not measured), 700 ppm byweight of acetic acid, 0.50% by weight (5000 ppm by weight) of hydrogen,41% by weight of carbon monoxide, 5% by weight of carbon dioxide, 5% byweight of methane, 5% by weight of nitrogen, 6% by weight (6 vol %) ofoxygen, and other minor components (total: 100% by weight). Thecondensate of the overhead was liquid-liquid separated in a decanter toform an aqueous phase (light phase) and an organic phase (heavy phase),and 1.4 parts by weight of the aqueous phase and 30 parts by weight ofthe organic phase were recycled to the reactor. The bottom stream (3parts by weight) from the first distillation column was recycled to thereactor, and the remainder of the feed (volatile phase) was withdrawn asa side-cut stream from the first distillation column. The compositionsof these process streams (the aqueous phase, the organic phase, and theside-cut stream) were substantially the same as those in ComparativeExample 13.

Example 21

A corrosion test was performed in the same manner as in ComparativeExample 14 except that 1 part by weight of purge nitrogen containing 6%by weight of oxygen relative to 100 parts by weight of the feed amountof the side-cut stream of the first distillation column was supplied tothe gaseous phase side of the differential pressure type level gauge formeasuring a liquid level of the bottom liquid of the second distillationcolumn.

From the top of the second distillation column, 50 parts by weight of anoverhead was withdrawn. The overhead had a composition containing 7.5%by weight of methyl iodide (MeI), 4.9% by weight of methyl acetate (MA),4.0% by weight of water (H₂O), 10 ppm by weight of hydrogen iodide (HI),0 ppm by weight of hydrogen, 0.00010% by weight (1.0 ppm by weight) ofcarbon monoxide, 0 ppm by weight of carbon dioxide, 0 ppm by weight ofmethane, 2% by weight of nitrogen, 0.40% by weight (0.90 vol %) ofoxygen, and other minor components, and the remainder was acetic acid.

The overhead from the second distillation column was cooled in acondenser, and the resulting condensate was held in a reflux tank. Aportion (32 parts by weight) of the condensate in the tank was withdrawnand was recycled to the reaction system. A portion (16 parts by weight)of the condensate was returned to the second distillation column forreflux at a reflux ratio of 0.5. The condensate had a compositioncontaining 7.7% by weight of methyl iodide (MeI), 5.0% by weight ofmethyl acetate (MA), 4.1% by weight of water (H₂O), 9 ppm by weight ofhydrogen iodide (HI), 0.00020% by weight (2.0 ppm by weight) of oxygen,and other minor components, and the remainder was acetic acid. From thecondenser, 1 part by weight of a noncondensable gas was withdrawn. Thenoncondensable gas had a composition having 7% by weight (6 vol %) ofoxygen and 93% by weight (94 vol %) of nitrogen, and the negligibleamounts of other components. A bottom stream (crude acetic acid) fromthe second distillation column had a composition containing 4 ppb byweight of methyl iodide (MeI), 0.05% by weight of water (H₂O), 5 ppb byweight of hydrogen iodide (HI), 5 ppm by weight of methyl acetate (MA),and other minor components (containing oxygen), and the remainder wasacetic acid.

Example 22

In the process for continuously producing acetic acid as shown in FIG.1, methanol was allowed to react with carbon monoxide (carbon monoxidehaving a concentration of oxygen of 2% by weight (2 vol %)) in acarbonylation reactor, the reaction mixture was continuously fed fromthe reactor to a flasher for flash evaporation to form a less-volatilephase (a bottom fraction at least containing a rhodium catalyst, lithiumiodide, acetic acid, methyl acetate, methyl iodide, water, and hydrogeniodide) and a volatile phase (liquid temperature of liquefied gaseousfraction: 140° C.). The volatile phase contained 27.1% by weight ofmethyl iodide (MeI), 4.5% by weight of methyl acetate (MA), 2.0% byweight of water (H₂O), 500 ppm by weight of hydrogen iodide (HI), 63.5%by weight of acetic acid, 0.0070% by weight (70 ppm by weight) ofhydrogen, 2% by weight of carbon monoxide, 0.060% by weight (600 ppm byweight) of carbon dioxide, 0.070% by weight (700 ppm by weight) ofmethane, 0.070% by weight (700 ppm by weight) of nitrogen, 0.30% byweight (0.70 vol %) of oxygen, and other minor components (total: 100%by weight).

The volatile phase (100 parts by weight) was fed to the firstdistillation column (actual number of plates: 20, feed plate: the 2ndplate from the bottom) and was distilled at a gauge pressure of 150 kPa,a column bottom temperature of 143° C., a column top temperature of 115°C., and a light-phase reflux ratio of 12. The resulting overhead fromthe column was cooled in a condenser to form a condensate and anoncondensable gas. The condensate (temperature: 40° C.) wasliquid-liquid separated in a decanter to form an aqueous phase (lightphase) and an organic phase (heavy phase), and 1.3 parts by weight ofthe aqueous phase (light phase) and 30 parts by weight of the organicphase (heavy phase) were recycled to the reactor. From the condenser,4.1 parts by weight of the noncondensable gas (off-gas stream) waswithdrawn. The overhead (column top) composition from the firstdistillation column (the composition of the overhead) was as follows:52.4% by weight of methyl iodide (MeI), 9.1% by weight of methyl acetate(MA), 25.6% by weight of water (H₂O), 100 ppm by weight of hydrogeniodide (HI), 7.1% by weight of acetic acid, 0.010% by weight (100 ppm byweight) of hydrogen, 5% by weight of carbon monoxide, 0.12% by weight(1200 ppm by weight) of carbon dioxide, 0.14% by weight (1400 ppm byweight) of methane, 0.14% by weight (1400 ppm by weight) of nitrogen,0.50% by weight (0.70 vol %) of oxygen, and other minor components(total: 100% by weight); and the composition of the noncondensable gas(off-gas stream) from the condenser was as follows: 15% by weight ofmethyl iodide (MeI), 1% by weight of methyl acetate (MA), 200 ppm byweight of water (H₂O), hydrogen iodide (HI) (not measured), 200 ppm byweight of acetic acid, 0.20% by weight (2000 ppm by weight) of hydrogen,71% by weight of carbon monoxide, 2% by weight of carbon dioxide, 2% byweight of methane, 2% by weight of nitrogen, 6% by weight (6 vol %) ofoxygen, and other minor components (total: 100% by weight). Thecomposition of the aqueous phase (light phase) was as follows: 3.3% byweight of methyl iodide (MeI), 6.6% by weight of methyl acetate (MA),73.0% by weight of water (H₂O), 100 ppm by weight of hydrogen iodide(HI), 17.0% by weight of acetic acid, 0.0014% by weight (14 ppm byweight) of oxygen, and other minor components (total: 100% by weight);and the composition of the organic phase (heavy phase) was as follows:86% by weight of methyl iodide (MeI), 11.4% by weight of methyl acetate(MA), 0.6% by weight of water (H₂O), 100 ppm by weight of hydrogeniodide (HI), 1.9% by weight of acetic acid, 0.0016% by weight (16 ppm byweight) of oxygen, and other minor components (total: 100% by weight).

A side-cut stream (62.8 parts by weight) of the first distillationcolumn was fed to a second distillation column for dehydration andpurification. The composition of the above side-cut stream was asfollows: 2.4% by weight of methyl iodide (MeI), 1.6% by weight of methylacetate (MA), 1.3% by weight of water (H₂O), 48 ppm by weight ofhydrogen iodide (HI), 94.6% by weight of acetic acid, 0.0010% by weight(10 ppm by weight) of oxygen, and other minor components (total: 100% byweight). The remainder of the feed (volatile phase) was recycled as abottom stream to the reaction system.

In such a continuous reaction process, the above-mentioned test pieceswere placed on the feed plate of the first distillation column (the 2ndplate from the bottom, temperature: 140° C.) and the upper part of thecolumn (the 19th plate from the bottom). After the process was operatedfor 500 hours, each test piece was examined for a corrosion test. Theweight of each test piece before and after the corrosion test wasmeasured to determine a corrosion amount.

Moreover, the crude acetic acid (side-cut stream) from the firstdistillation column was examined for the APHA.

Tables 62 to 66 show the compositions (component ratios) and the resultsof the corrosion test. Incidentally, Table 62 shows the compositions ofthe liquid phases in Comparative Examples 1 to 12, Table 63 shows thecompositions of the liquid phases in Examples 1 to 19, Table 64 showsthe compositions of the gaseous phases in Comparative Examples 1 to 12and Examples 1 to 19, and Table 65 and Table 66 show the results of thecorrosion test. In Table 62 to Table 64, “wt %” represents % by weight,“vol %” denotes % by volume, “MeI” represents methyl iodide, “HI”denotes hydrogen iodide, “MA” represents methyl acetate, “MeOH” denotesmethanol, “Ac” represents acetic acid, “AD” denotes acetaldehyde, “PA”represents propionic acid, and “LiI” denotes lithium iodide.

In Comparative Examples and Examples conducted under a concentration ofoxygen of 1 to 7 vol % in a gaseous phase, the oxygen concentrationafter the corrosion test was reduced by about 0 to 2 vol %. In examplesconducted under a concentration of oxygen of less than 1% in a gaseousphase, the oxygen concentration after the corrosion test was reduced byabout 2 to 10 vol %.

Since it is difficult to measure the concentration of DME, which has alow boiling point, the concentration of DME was determined to be thecalculation concentration of DME fed.

In replacing oxygen in the liquid with nitrogen gas by nitrogen gasbubbling after feeding of the liquid, a portion of components in themixture was discharged to the outside of the system in accompanying withnitrogen gaseous phase. Thus, there is a difference, particularly in aconcentration of methyl iodide, between the feed composition and thecomposition after the experiment.

Incidentally, for all Comparative Examples and Examples except forComparative Examples 8, 9 and Examples 8, 9, a peak originated from DMEin the liquid was observed by gas chromatography after finishing of theexperiment. The concentration of DME calculated from the percentage ofarea was about 10 to 1000 ppm by weight.

TABLE 62 Temper- MeI Water HI MA MeOH DME Ac AD PA LiI ature Pressure wt% wt % wt % wt % wt % wt % wt % wt % wt % wt % ° C. KPG Com. Ex. 1 10.92.1 0.1 2.8 0.1 0.01 69   0.02 0.01 15.3 190 2800 Com. Ex. 2 0.8 3.20.01 1.1 0.1 0.01 Remainder 0.005 0.01 19.7 140 140 Com. Ex. 3 32.1 2.20.1 5.5 0.1 0.01 Remainder 0.08 0.01 0.01 140 130 Com. Ex. 4 60.5 23.10.1 10.5 0.1 0.01 Remainder 0.19 0.01 less than 116 130 detection limit(less than 1 ppb) Com. Ex. 5 0.9 1.1 0.01 0.8 0 0.01 Remainder 0 0.010.4 140 130 Com. Ex. 6 2.9 1.5 0.01 2.2 0.01 0.01 Remainder 0.01 0.010.0003 136 140 Com. Ex. 7 8.4 4.9 0.01 6.5 0.1 0.01 Remainder 0.00030.01 less than 150 200 detection limit (less than 1 ppb) Com. Ex. 8 95ppb  0.2 23 ppb 18 ppm  0 0 Remainder 0 0 less than 137 80 detectionlimit (less than 1 ppb) Com. Ex. 9 6 ppb 0.05  4 ppb 5 ppm 0 0 Remainder0 0.01 0.0002 160 230 Com. Ex. 10 Remainder 1.1 0.01 9.7 0.01 0.01 2.10.05 0 less than 80 250 detection limit (less than 1 ppb) Com. Ex. 11 1ppb 0.05 0.7 ppb  9 ppm less than less than Remainder 0.8 ppm 0.01 lessthan 138 80 detection detection detection limit (less limit (less limit(less than 1 ppm) than 1 ppm) than 1 ppb) Com. Ex. 12 3 1.5 0.01 2 0.010.01 Remainder 0.01 0.01 0.0003 136 140

TABLE 63 Temper- MeI Water HI MA MeOH DME Ac AD PA LiI ature Pressure wt% wt % wt % wt % wt % wt % wt % wt % wt % wt % ° C. KPG Ex. 1 10.5 2.20.1 3 0.1 0.01 Remainder 0.02 0.01 14.9 190 2800 Ex. 2 0.9 3 0.01 1.20.1 0.01 Remainder 0.004 0.01 20.1 140 140 Ex. 3 32.8 1.9 0.1 6.1 0.10.01 Remainder 0.09 0.01 0.01 140 130 Ex. 4 59.9 22.8 0.1 11 0.1 0.01Remainder 0.2 0.01 less than 116 130 detection limit (less than 1 ppb)Ex. 5 1.1 0.9 0.01 1.1 0 0.01 Remainder 0 0.01 0.4 140 130 Ex. 6 3.2 1.80.01 2 0.01 0.01 Remainder 0.008 0.01 0.0003 136 140 Ex. 7 7.9 4.7 0.016 0.1 0.01 Remainder 0.0003 0.01 less than 150 200 detection limit (lessthan 1 ppb) Ex. 8 102 ppb  0.18  19 ppb 23 ppm  0 0 Remainder 0 0 lessthan 137 80 detection limit (less than 1 ppb) Ex. 9   5 ppb 0.04   6 ppb5 ppm 0 0 Remainder 0 0.01 0.0002 160 230 Ex. 10 Remainder 0.8 0.01 10.10.008 0.01 1.9 0.08 0 less than 80 250 detection limit (less than 1 ppb)Ex. 11 2.9 1.3 0.01 1.8 0.011 0.01 Remainder 0.009 0.01 0.0003 136 140Ex. 12 3.2 1.6 0.01 1.9 0.008 0.01 Remainder 0.01 0.01 0.0003 136 140Ex. 13 3.1 1.4 0.01 2.1 0.012 0.01 Remainder 0.011 0.01 0.0003 136 140Ex. 14 2.8 1.5 0.01 2 0.01 0.01 Remainder 0.008 0.01 0.0003 136 140 Ex.15 3.3 1.6 0.01 2 0.007 0.01 Remainder 0.011 0.01 0.0003 136 140 Ex. 163 1.2 0.01 2.1 0.09 0.01 Remainder 0.008 0.01 0.0003 136 140 Ex. 17 3.31.5 0.01 1.9 0.012 0.01 Remainder 0.012 0.01 0.0003 136 140 Ex. 18 0.9ppb 0.05 0.8 ppb 8 ppm less than less than Remainder   1 ppm 0.01 lessthan 138 80 detection detection detection limit (less limit (less limit(less than 1 ppm) than 1 ppm) than 1 ppb) Ex. 19 1.1 ppb 0.05 0.9 ppb 7ppm less than less than Remainder 1.1 ppm 0.01 less than 138 80detection detection detection limit (less limit (less limit (less than 1ppm) than 1 ppm) than 1 ppb)

TABLE 64 H₂ CO CO₂ CH₄ N₂ O₂ H₂ CO CO₂ CH₄ N₂ O₂ vol % vol % vol % vol %vol % vol % kPa kPa kPa kPa kPa kPa Com. Ex. 1 0 93 0 0 0 7 0 94 0 0 07.09 Com. Ex. 2 0 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 3 0 93 0 0 0 7 094 0 0 0 7.09 Com. Ex. 4 0 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 5 0 93 00 0 7 0 94 0 0 0 7.09 Com. Ex. 6 0 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 70 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 8 0 93 0 0 0 7 0 94 0 0 0 7.09Com. Ex. 9 0 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 10 0 93 0 0 0 7 0 94 00 0 7.09 Com. Ex. 11 0 93 0 0 0 7 0 94 0 0 0 7.09 Com. Ex. 12 0  0 0 093  7 0 0 0 0 94 7.09 Ex. 1 0 Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex.2 0 Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 3 0 Remainder 0 0 0 0.0100 101 0 0 0 0.01 Ex. 4 0 Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 5 0Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 6 0 Remainder 0 0 0 0.010 0101 0 0 0 0.01 Ex. 7 0 Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 8 0Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 9 0 Remainder 0 0 0 0.010 0101 0 0 0 0.01 Ex. 10 0 Remainder 0 0 0 0.010 0 101 0 0 0 0.01 Ex. 11 095 0 0 0 5 0 96 0 0 0 5.07 Ex. 12 0 99 0 0 0 1 0 100 0 0 0 1.01 Ex. 13 0Remainder 0 0 0 0.00010 0 101 0 0 0 0.00 Ex. 14 0  0 0 0 Remainder 0.100 0 0 0 101 0.10 Ex. 15 0  0 0 0 Remainder 0.010 0 0 0 0 101 0.01 Ex. 160  0 0 0 Remainder 0.0010 0 0 0 0 101 0.001 Ex. 17 2 Remainder 15 7 80.010 2 69 15 7 8 0.00 Ex. 18 0 Remainder 0 0 0 0.00010 0 101 0 0 0 0.00Ex. 19 0 Remainder 0 0 0 0.000010 0 101 0 0 0 0.00

TABLE 65 Corrosion test results Zr HB2 HC276 SUS316 Partial PartialPartial Partial mm/Y corrosion mm/Y corrosion mm/Y corrosion mm/Ycorrosion APHA Com. Ex. 1 0.000 none 0.1 present — — — — 500< Com. Ex. 20.000 none 0.1 present — — — — 500< Com. Ex. 3 0.000 none 0.05 present0.02 present 0.06 none 500< Com. Ex. 4 0.000 none 0.08 present 0.027present 0.09 none 500< Com. Ex. 5 0.000 none 0.12 present 0.02 present0.11 none 500< Com. Ex. 6 0.000 none 0.15 present 0.12 present — — 500<Com. Ex. 7 0.000 none 0.19 present 0.28 present — — 500< Com. Ex. 80.000 none 0.05 present 0.03 none 0.19 none 80 Com. Ex. 9 0.000 none0.09 present 0.02 present 0.08 present 90 Com. Ex. 10 0.000 none 0.04present 0.03 present 0.01 present 500< Com. Ex. 11 0.000 none 0.08present 0.03 present 0.02 — 70 Com. Ex. 12 0.000 none 0.29 present 0.3present — — 500< Com. Ex. 13 Feed plate 0.000 none 0.05 present 0.02present 0.09 none Upper part 0.000 none 0.06 present 0.01 present 0.06none 500< of column Com. Ex. 14 2nd plate 0.000 none 0.17 present 0.21present — — 500< from bottom Upper part 0.000 none 0.09 present 0.2present — — of column

TABLE 66 Corrosion test results Zr HB2 HC276 SUS316 Partial PartialPartial Partial mm/Y corrosion mm/Y corrosion mm/Y corrosion mm/Ycorrosion APHA Ex. 1 0.000 none 0.01 none — — — — 60 Ex. 2 0.000 none0.001 none — — — — 50 Ex. 3 0.000 none 0.005 none 0.01 none 0.06 none 50Ex. 4 0.000 none 0.015 none 0.025 none 0.10 none 25 Ex. 5 0.000 none0.01 none 0.02 none 0.10 none 25 Ex. 6 0.000 none 0.03 none 0.09 none —— 25 Ex. 7 0.000 none 0.07 none 0.25 none — — 35 Ex. 8 0.000 none 0.001none 0.01 none 0.05 none 10 Ex. 9 0.000 none 0.000 none 0.005 none 0.03none 10 Ex. 10 0.000 none 0.001 none 0.02 none 0.13 none 40 Ex. 11 0.000none 0.08 none 0.19 none — — 50 Ex. 12 0.000 none 0.05 none 0.11 none —— 40 Ex. 13 0.000 none 0.03 none 0.09 none — — 20 Ex. 14 0.000 none 0.06none 0.1 none — — 40 Ex. 15 0.000 none 0.04 none 0.09 none — — 30 Ex. 160.000 none 0.03 none 0.10 none — — 25 Ex. 17 0.000 none 0.03 none 0.11none — — 35 Ex. 18 0.000 none 0.000 none 0.000 none 0.05 — 5 Ex. 190.000 none 0.000 none 0.000 none 0.04 — 5 Ex. 20 Feed plate 0.000 none0.005 none 0.02 none — — 40 Upper part 0.000 none 0.002 none 0.01 none —— of column Ex. 21 2nd plate 0.000 none 0.01 none 0.19 none — — 10 frombottom Upper part 0.000 none 0.009 none 0.18 none — — of column Ex. 222nd plate 0.000 none 0.02 none 0.02 none 0.07 — 80 from bottom Upperpart 0.000 none 0.03 none 0.01 none 0.06 — of column

From the results shown in Tables 62 to 66, the following are seen.

Carbon monoxide CO reduces the concentration of oxygen by the reductionreaction: CO+½O₂→CO₂ to form what is called a reducing atmosphere.However, a high concentration of oxygen may not form a reducingatmosphere in some cases. From the comparison of Comparative Examples 1to 11 with Examples 1 to 11, different in the liquid composition, underthe condition of the oxygen concentration of 7 vol % in ComparativeExamples, the test piece HB2, free from Cr and weak under an oxidizingatmosphere, had an increased corrosion rate. For the SUS316 material,the corrosion rate in Comparative Example 11 tended to slightly decreasecompared with that in each of Examples 18 and 19. Such a tendency seemsto be one of general characteristics of SUS, and the corrosion rate maytend to be increased adversely in an excessively high reducingatmosphere. Thus, it was found that, according to the condition, thepresence of some oxygen in the process sometimes reduced the corrosion.

Moreover, in the test piece HB2 or other materials in ComparativeExamples, pitting corrosion or spot corrosion was often observed whichseems to be the influence of iodine I₂ produced by the reaction:2HI+½O₂→I₂+H₂O or other reactions. In the test pieces SUS316 and HC276containing Cr, under a condition of not a very high oxygen concentrationsome increase in corrosion rate was observed although such an increasedid not lead the test pieces to excessive corrosion.

Further, the solution after the corrosion test had an apparently largeAPHA value compared with the solution before the corrosion test tobecome dark to reddish brown color peculiar to iodine. This coloring iscaused by production of iodine I₂. The APHA value shown in Tables is upto 500. In this regard, in experimental examples in which the oxygenconcentration was high and iodine I₂ was produced at a highconcentration, the solution had little or no transparency and colored toa degree not expressed by APHA. This event shows that iodine I₂ flowsout to the succeeding step in a process having a high oxygenconcentration, resulting in the acceleration of corrosion in thesucceeding step and the coloring or the increase in a total iodineconcentration due to contamination with iodine in a product.

Comparative Example 12 is an experiment carried out under a nitrogenatmosphere which has no reaction reducing the concentration of oxygenO₂, different from a carbon monoxide CO. Thus, the test pieces(particularly, test piece HB2) had a significantly increased corrosionrate compared with Example 6.

In Example 11, the concentration of oxygen was reduced to half (1 vol %)of that in Comparative Example 11. Although there is still an adverseinfluence of oxygen on the corrosion test and the coloring, theseinfluences are considerably small compared with Comparative Example.Thus, the above oxygen concentration is not an unacceptableconcentration.

In Example 12, the concentration of oxygen was reduced to ¼ (0.5 vol %)of that in Comparative Example 11. Although there is still an adverseinfluence of oxygen on the corrosion test and the coloring, theseinfluences are considerably small compared with Comparative Example.Thus, the above oxygen concentration is also not an unacceptableconcentration.

In Example 13, the concentration of oxygen O₂ was reduced as low aspossible as long as the concentration was measurable. The results ofExample 13 are substantially equivalent to those of Example 6, in whichthe concentration of oxygen was 0.01 vol %. The results show that theconcentration of oxygen reduced to some degree or extent has little orno adverse influence and show substantially equivalent behaviortherebetween.

Incidentally, in Examples 18 and 19, the corrosion rate of the testpiece SUS316 was slightly high compared with that in Comparative Example11. The reason why is as follows. SUS316 sometimes tend to increase incorrosion rate under a high reducing atmosphere, and thus, under less orno oxygen such as the condition of Examples 18 and 19, corrosion isaccelerated. Since this tendency seems to be observed markedly under alower concentration of oxygen, it is not advisable, particularly forstainless steel (SUS)-based material, to reduce the concentration ofoxygen to sufficiently near zero (for example, 1 vol ppt, 1 vol ppb)under a reducing condition in the presence of carbon monoxide CO.However, such a degree of corrosion was not unacceptable level at all.Incidentally, under such a condition, impurities have little or noiodine contents and the oxygen concentration is extremely low, and thusless- or no-colored product acetic acid is obtained.

In Example 14, the concentration of oxygen was reduced to 0.1 vol %under a nitrogen gas N₂ atmosphere. According to Example 14, althoughthe test piece HB2 is slightly corroded which results in slightcoloring, the test piece HB2 has a reduced corrosion compared withExample 11, and the concentration of oxygen is not an unacceptableconcentration.

In Examples 15 and 16, the concentration of oxygen was further reducedunder a nitrogen gas N₂ atmosphere. In Examples 15 and 16, includingExample 13, the degree of the corrosion is not very different from thedegree of the corrosion under a carbon monoxide CO atmosphere. Under alower concentration of oxygen, the corrosion rate and the coloring weresubstantially the same level as those under a carbon monoxide COatmosphere.

Example 17 is an experiment using a different feed gas. As compared withExample 6, it is found that different feed gases, each having sufficientcarbon monoxide CO gas and the same concentration of oxygen, providesubstantially the same corrosion rate and APHA.

From the comparison of Comparative Example 13 with Example 20, whencarbon monoxide fed to the reactor has a high concentration of oxygen,the test piece, particularly the test piece HB2 weak in oxygen, iscorroded and the side-cut liquid has a high degree of coloring (largeAPHA value). Moreover, in Comparative Example 13, due to a considerablyhigh concentration of oxygen in the upper part of the column, the testpiece HB2 disposed at the upper part of the first distillation columnhas a large corrosion rate in spite of a low temperature thereof,compared with the test piece HB2 disposed at the feed plate. The testpiece HB2 usually shows a high corrosion resistance under a lowconcentration of oxygen. Thus, the corrosion rate of the test piece HB2under a low oxygen concentration condition as Example 20 is smallcompared with that under an overall high concentration of oxygen, whilethe corrosion rate test piece HB2 at the feed plate having a highertemperature is larger than that at the upper part of the column having alower temperature.

Comparative Example 14 and Example 21 had the same tendency as theComparative Example 13 and Example 20, although the feed composition isdifferent.

Generally, the price of the material is low in this order ofZr>HB2>HC>SUS.

Considering the price, the material can be selected on the basis of thecorrosion rate as the following standards, although the thickness of thematerial, the frequency of renovation, or other factors are influenced.

Corrosion rate of 0.05 mm/Y or less: suitable for use

0.05 to 0.1 mm/Y: usable level

0.1 mm/Y to 0.2 mm/Y: usable depending on conditions

0.2 mm/Y or more: unusable

INDUSTRIAL APPLICABILITY

The present invention effectively prevents the corrosion of the processunit and/or line and is significantly useful as a process for stablyproducing high-quality acetic acid.

REFERENCE SIGNS LIST

-   -   (1) . . . Reactor    -   (2) . . . Evaporator    -   (3) . . . First distillation column    -   (5) . . . Second distillation column (Dehydration column)    -   (6) . . . Third distillation column (Heavy end column)    -   (7) . . . Fourth distillation column (Purification column)    -   (8) . . . Ion exchange tank    -   (10) . . . Decanter    -   (11) . . . Fifth distillation column (First aldehyde-removing        column)    -   (12) . . . Sixth distillation column (Water extractive        distillation column)    -   (13) . . . Seventh distillation column (Second aldehyde-removing        column)    -   (14) . . . Eighth distillation column (Alkane-removing column)    -   (16) . . . High-pressure absorption column    -   (17) . . . Low-pressure absorption column    -   (18) . . . Diffusion column

1. A process for producing acetic acid, comprising: (1) allowingmethanol to carbonylation react with carbon monoxide in the presence ofa catalyst system, acetic acid, methyl acetate, and water, wherein thecatalyst system comprises a metal catalyst, an ionic metal iodide, andmethyl iodide; (2) separating the reaction mixture into a volatile phaseand a less-volatile phase; (3) distilling the volatile phase to form afirst overhead and an acetic acid stream, wherein the first overhead isrich in at least one lower boiling component selected from the groupconsisting of methyl iodide and acetaldehyde, and the acetic acid streamis rich in acetic acid; and at least one step group selected from thegroup consisting of the following step groups (4), (9), and (15): (4) apurification step group for obtaining purified acetic acid from theacetic acid stream; (9) a separation step group for separating at leastacetaldehyde from the first overhead; and (15) an off-gas treatment stepgroup for absorption-treating an off-gas from the process with anabsorption solvent and forming a carbon monoxide-rich stream and anacetic acid-rich stream; wherein the process is operated under at leastone selected from the group consisting of the following conditions (a)and (b): (a) the concentration of oxygen in a gaseous phase of theprocess is less than 7% by volume, (b) the concentration of oxygen in aliquid phase of the process is less than 7×10⁻⁵ g/g.
 2. The processaccording to claim 1, wherein the gaseous phase of the process containsat least one member selected from the group consisting of methyl iodideand hydrogen iodide.
 3. The process according to claim 1, wherein thegaseous phase of the process further contains at least one memberselected from the group consisting of acetic acid, methyl acetate,methanol, water, acetaldehyde, a by-product derived from acetaldehyde,and a dialkyl ether; the by-product contains at least one memberselected from the group consisting of an alkyl iodide with 2 or morecarbon atoms, an alkanal with 4 or more carbon atoms, analkanecarboxylic acid with 3 or more carbon atoms, an alkane, and aketone; and the dialkyl ether contains at least dimethyl ether.
 4. Theprocess according to claim 1, wherein, in at least one process streamselected from the group consisting of a stream of a process unit and astream of a process line, the process is operated under at least oneselected from the group consisting of the following conditions (a-1) and(b-1): (a-1) the concentration of oxygen in the gaseous phase is 5% byvolume or less, (b-1) the concentration of oxygen in the liquid phase is2×10⁻⁵ g/g or less.
 5. The process according to claim 1, wherein, in atleast one process stream selected from the group consisting of a streamof a process unit and a stream of a process line, a ratio of oxygenrelative to carbon monoxide in each of the gaseous phase and the liquidphase is 2% by volume or less.
 6. The process according to claim 1,wherein, in at least one process stream selected from the groupconsisting of a stream of a process unit and a stream of a process line,a ratio of oxygen relative to carbon monoxide in each of the gaseousphase and the liquid phase is 1% by volume or less.
 7. The processaccording to claim 1, wherein at least one component selected from thegroup consisting of an oxygen-containing gas, an oxygen-containingcompound, and an oxygen generator is introduced to the process; and inat least one process stream selected from the group consisting of astream of a process unit and a stream of a process line, theconcentration of oxygen in the gaseous phase is 1 ppt by volume or more,and/or the concentration of oxygen in the liquid phase is 0.1×10⁻⁹ g/gor more.
 8. The process according to claim 1, wherein the concentrationof oxygen in the gaseous phase is 1 ppb by volume or more.
 9. Theprocess according to claim 1, wherein a concentration of oxygen in atleast one process stream selected from the group consisting of thegaseous phase and the liquid phase is 0.25 mol or less relative to 1 molof a total amount of hydrogen iodide and methyl iodide.
 10. The processaccording to claim 1, wherein the purification step group (4) comprisesat least (5) a dehydration step among the following steps (5) to (8):(5) dehydrating the acetic acid stream; (6) removing a higher boilingcomponent from the acetic acid stream; (7) furtherpurification-distilling an acetic acid stream from the step (6); and (8)ion-exchange separating an iodine compound from an acetic acid streamfrom the step (7).
 11. The process according to claim 1, wherein theseparation step group (9) comprises at least steps (10) to (13) amongthe following steps (10) to (14): (10) condensing the first overhead toform two liquid phases with an upper phase and a lower phase; (11)forming a fifth overhead from the upper phase, the lower phase, or both,wherein the fifth overhead is rich in acetaldehyde and methyl iodide;(12) extracting acetaldehyde from the fifth overhead to form an extractand a raffinate, wherein the extract is rich in acetaldehyde and theraffinate is rich in methyl iodide; (13) separating an aldehyde from theextract, the raffinate, or both; and (14) separating an alkane from theupper phase, the lower phase, or both.
 12. The process according toclaim 1, wherein the off-gas treatment step group (15) comprises atleast one absorption step selected from the group consisting of steps(16) and (17) among the following steps (16) to (18): (16) absorbing theoff-gas to an absorption solvent at a high pressure; (17) absorbing theoff-gas to an absorption solvent at a low pressure; and (18) diffusing agaseous component absorbed in the absorption steps (16) and (17). 13.The process according to claim 1, wherein the gaseous phase of theprocess comprises an off-gas from the process.
 14. A process accordingto claim 1, wherein the process is operated under at least one selectedfrom the group consisting of the conditions (a) and (b) recited in claim1 to produce a product acetic acid having an iodine (12) concentrationof not more than 10 ppb by weight, or a hydrogen iodide concentration ofnot more than 20 ppb by weight, or an iodine (12) concentration of notmore than 10 ppb by weight and a hydrogen iodide concentration of notmore than 20 ppb by weight.
 15. The process according to claim 2,wherein the product acetic acid has a hydrogen iodide concentration ofnot more than 10 ppb by weight.
 16. A method for reducing formation ofiodine in a process, comprising: (1) allowing methanol to carbonylationreact with carbon monoxide in the presence of a catalyst system, aceticacid, methyl acetate, and water, wherein the catalyst system comprises ametal catalyst, an ionic metal iodide, and methyl iodide; (2) separatingthe reaction mixture into a volatile phase and a less-volatile phase;(3) distilling the volatile phase to form a first overhead and an aceticacid stream, wherein the first overhead is rich in at least one lowerboiling component selected from the group consisting of methyl iodideand acetaldehyde, and the acetic acid stream is rich in acetic acid; andat least one step group selected from the group consisting of thefollowing step groups (4), (9), and (15): (4) a purification step groupfor obtaining purified acetic acid from the acetic acid stream; (9) aseparation step group for separating at least acetaldehyde from thefirst overhead; and (15) an off-gas treatment step group forabsorption-treating an off-gas from the process with an absorptionsolvent and forming a carbon monoxide-rich stream and an aceticacid-rich stream; wherein the process is operated under at least oneselected from the group consisting of the following conditions (a) and(b): (a) the concentration of oxygen in a gaseous phase portion of theprocess is less than 7% by volume, (b) the concentration of oxygen in aliquid stream of the process is less than 7×10⁻⁵ g/g.
 17. The methodaccording to claim 16, wherein the process is operated under at leastone selected from the group consisting of the conditions (a) and (b)recited in claim 16 to produce a product acetic acid having an iodine(I₂) concentration of not more than 10 ppb by weight, or a hydrogeniodide concentration of not more than 20 ppb by weight, or an iodine(I₂) concentration of not more than 10 ppb by weight and a hydrogeniodide concentration of not more than 20 ppb by weight.
 18. The methodaccording to claim 17, wherein the product acetic acid has a hydrogeniodide concentration of not more than 10 ppb by weight.